Remove database setup duplication

The Database fixture was being used twice. This removes is from the
test_v3.RestfulTestCase class in favor of keeping the setup in the
rest.RestfulTestCase class. To do this the rest.RestfulTestCase class
needed a method to define extensions.

implements bp tests-on-rdbmses
Change-Id: Ia1dfd887e23b0055fdc260473be367fbcd6c7662
Merge "Remove database setup duplication"