Make logging level in unit tests configurable.

Added a debug option to unittest(BaseTestCase) which sets logging level of
fixtures.FakeLogger to DEBUG. To enable this option, set OS_DEBUG=True or
OS_DEBUG=1 in environment variables.

Fixes: bug #1181925
Change-Id: Id09008ddabb837290edf12e1cbd014dc74760113
Merge "Make logging level in unit tests configurable."