Remove unneeded context initialization in tests

Some test case classes that inherit from
ironic.tests.db.base.DbTestCase set context in their setUp method,
while it is already initialized in DbTestCase's setUp method. This
change removes such unneeded initializations.

Closes-bug: #1373979
Change-Id: I7ab07168f1f30b4544801a1798cbda191c8d15c7
Merge "Remove unneeded context initialization in tests"