Make SQL the default backend for Identity & Assignment unit tests.

We have a goal to deprecate the kvs backends for Identity and
Assignment. Before we can do this, we need to ensure our unit
tests are not dependant on them. This patch sets the Identity
backend for unit tests to SQL. Since, by default, the Assignment
backend will pick the same backend type as Identity, this also
means that it will follow suit.

As well as ensuring correct database initialization, this patch
moves a test that was erronously in v2_auth testing to v3. The
test was passing before due to the fact that the kvs assignment
driver does not correctly honor the domain boundary for all
project APIs.

Change-Id: I7b6e753b379a43d09ad8ea077ed6796d1f2e9a26
Partially-Closes: bug 1077282
Merge "Make SQL the default backend for Identity & Assignment unit tests."