Self-signed certs should not kill tests

When running against an endpoint with a self-signed cert, we should be able to instruct the base rest_client to ignore cert validation, otherwise, Tempest returns a whole bunch of these:

======================================================================
ERROR: Should not be able to create volume without passing size
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/nose/tools.py", line 80, in newfunc
    func(*arg, **kw)
  File "/opt/tempest/tempest/tests/volume/test_volumes_negative.py", line 84, in test_create_volume_with_out_passing_size
    metadata=metadata)
  File "/opt/tempest/tempest/services/volume/json/volumes_client.py", line 83, in create_volume
    resp, body = self.post('volumes', post_body, self.headers)
  File "/opt/tempest/tempest/common/rest_client.py", line 171, in post
    return self.request('POST', url, headers, body)
  File "/opt/tempest/tempest/common/rest_client.py", line 211, in request
    headers=headers, body=body)
  File "/usr/local/lib/python2.7/dist-packages/httplib2/__init__.py", line 1597, in request
    (response, content) = self._request(conn, authority, uri, request_uri, method, body, headers, redirections, cachekey)
  File "/usr/local/lib/python2.7/dist-packages/httplib2/__init__.py", line 1345, in _request
    (response, content) = self._conn_request(conn, request_uri, method, body, headers)
  File "/usr/local/lib/python2.7/dist-packages/httplib2/__init__.py", line 1281, in _conn_request
    conn.connect()
  File "/usr/local/lib/python2.7/dist-packages/httplib2/__init__.py", line 1036, in connect
    raise SSLHandshakeError(e)
SSLHandshakeError: [Errno 1] _ssl.c:504: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

The regular clients allow this using httplib2.Http.__init__'s disable_ssl_certificate_verification to True.