convert to dh_python2

The attached patch converts image-store-proxy away from the deprecated python-central to dh_python2, which is a goal for Oneiric. See bug 788514.

However, when I tried to test-build this locally, I got the following test failures:

[FAIL]
Traceback (most recent call last):
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/lib/tests/mocker.py", line 102, in test_method_wrapper
    result = test_method()
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/tests/test_resources.py", line 341, in testCancel
    self.assertTrue(body.startswith('{"states":'), body)
  File "/usr/lib/python2.7/unittest/case.py", line 608, in deprecated_func
    return original_func(*args, **kwargs)
  File "/usr/lib/python2.7/unittest/case.py", line 420, in assertTrue
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: {"error-message": "Bad request signature"}

imagestore.tests.test_resources.ResourcesTest.testCancel
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/lib/tests/mocker.py", line 102, in test_method_wrapper
    result = test_method()
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/tests/test_resources.py", line 383, in testClearError
    self.assertTrue(body.startswith('{"states":'), body)
  File "/usr/lib/python2.7/unittest/case.py", line 608, in deprecated_func
    return original_func(*args, **kwargs)
  File "/usr/lib/python2.7/unittest/case.py", line 420, in assertTrue
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: {"error-message": "Bad request signature"}

imagestore.tests.test_resources.ResourcesTest.testClearError
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/lib/tests/mocker.py", line 102, in test_method_wrapper
    result = test_method()
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/tests/test_resources.py", line 410, in testImageUriEncodedWithUrlSafeBase64
    self.assertTrue(body.startswith('{"states":'), body)
  File "/usr/lib/python2.7/unittest/case.py", line 608, in deprecated_func
    return original_func(*args, **kwargs)
  File "/usr/lib/python2.7/unittest/case.py", line 420, in assertTrue
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: {"error-message": "Bad request signature"}

imagestore.tests.test_resources.ResourcesTest.testImageUriEncodedWithUrlSafeBase64
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/lib/tests/mocker.py", line 102, in test_method_wrapper
    result = test_method()
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/tests/test_resources.py", line 281, in testInstall
    self.assertTrue(body.startswith('{"states":'), body)
  File "/usr/lib/python2.7/unittest/case.py", line 608, in deprecated_func
    return original_func(*args, **kwargs)
  File "/usr/lib/python2.7/unittest/case.py", line 420, in assertTrue
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: {"error-message": "Bad request signature"}

imagestore.tests.test_resources.ResourcesTest.testInstall
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/lib/tests/mocker.py", line 102, in test_method_wrapper
    result = test_method()
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/tests/test_resources.py", line 314, in testInstallError
    self.assertStartsWith(body, '{"states":')
  File "/home/cjwatson/src/ubuntu/image-store-proxy/tmp/image-store-proxy-1.0.4/build/lib.linux-i686-2.7/imagestore/lib/tests/mocker.py", line 226, in failUnlessStartsWith
    (first, second))
twisted.trial.unittest.FailTest: '{"error-message": "Bad request signature"}' doesn't start with '{"states":'

imagestore.tests.test_resources.ResourcesTest.testInstallError
===============================================================================
[ERROR]
Traceback (most recent call last):
Failure: exceptions.ValueError: !IGNORED!

imagestore.tests.test_resources.ResourcesTest.testInstallWithBadSignature

Could you look into this, please?