Behavior of deleting non-existent keypairs changed

This change was caused by https://review.openstack.org/#/c/17285/ and caused these errors in tempest from
http://logs.openstack.org/17351/1/gate/gate-tempest-devstack-vm/21154/console.html.gz

17:39:13 ======================================================================
17:39:13 FAIL: Non-existant key deletion should throw a proper error
17:39:13 ----------------------------------------------------------------------
17:39:13 Traceback (most recent call last):
17:39:13 File "/opt/stack/tempest/tempest/tests/compute/keypairs/test_keypairs.py", line 148, in test_keypair_delete_nonexistant_key
17:39:13 self.fail('nonexistent key')
17:39:13 AssertionError: nonexistent key
17:39:13 """Fail immediately, with the given message."""
17:39:13 >> raise self.failureException('nonexistent key')
17:39:13
17:39:13
17:39:13 ======================================================================
17:39:13 FAIL: Non-existant key deletion should throw a proper error
17:39:13 ----------------------------------------------------------------------
17:39:13 Traceback (most recent call last):
17:39:13 File "/opt/stack/tempest/tempest/tests/compute/keypairs/test_keypairs.py", line 148, in test_keypair_delete_nonexistant_key
17:39:13 self.fail('nonexistent key')
17:39:13 AssertionError: nonexistent key
17:39:13 """Fail immediately, with the given message."""
17:39:13 >> raise self.failureException('nonexistent key')
17:39:13
17:39:13
17:39:13 ======================================================================
17:39:13 FAIL: A DELETE request for another user's keypair should fail
17:39:13 ----------------------------------------------------------------------
17:39:13 Traceback (most recent call last):
17:39:13 File "/usr/lib/python2.7/dist-packages/nose/tools.py", line 87, in newfunc
17:39:13 raise AssertionError(message)
17:39:13 AssertionError: test_delete_keypair_of_alt_account_fails() did not raise NotFound
17:39:13 'test_delete_keypair_of_alt_account_fails() did not raise NotFound' = "%s() did not raise %s" % ('test_delete_keypair_of_alt_account_fails', 'NotFound')
17:39:13 >> raise AssertionError('test_delete_keypair_of_alt_account_fails() did not raise NotFound')
17:39:13
17:39:13
17: