Check if creds are present and not None

Currently, KeystoneStrategy checks whether the required credentials are
present in the creds dict but it doesn't check whether they are valid or
not.

This patch checks whether the required creds are present and not None
otherwise a MissingCredentials exception will be raised.

Note: No need for checking parameters' values types since they'll be
instances of basestring once set otherwise they'll be None.

Fixes bug 1157765

Change-Id: I664a604c3cbf2fca60a88c4d887cd9a4b678c8a5
Add tests for image visibility filter in db

Some aspects of visibility filtering in the db api were not being
tested. This patch adds tests to ensure visibility=public|private is
working properly.

Change-Id: I67a4b8e33fab6f4f8f1ed4b05b2b8cac10153105
Merge "Add tests for image visibility filter in db"