Use DbTestCase as test base when context needed

There is tests.db.base.DbTestCase class which inherits from
tests.base.TestCase and adds context to test case object. Some test
cases initialize context in their setUp method, so to avoid this
initialization and unnecessary imports tests.db.base.DbTestCase can
be used as their base class.

Change-Id: I46cf7d1a33f8fbaa7d720bcfc574c9640ef83c06
Updated from global requirements

Change-Id: I05b251ce54af888229910eefb39db582c60c6d84
Replace custom lazy loading by stevedore

Change-Id: I0b09e8822af6e8c3ce7299d31eb53f30a50262bc
Merge "Use DbTestCase as test base when context needed"
Merge "Replace custom lazy loading by stevedore"