Fix overzealous conductor test for vol_usage_update

The rest of the conductor tests are pretty unit-esque, in that
they don't test the underlying functionality. Rather, they confirm
that the various conductor APIs (public, local, rpc, and manger)
all agree and that the plumbing between them works as expected.

The test_vol_usage_update() test does too much, and as a result,
gets all twisted in how it mucks with mox to stub out utcnow()
without breaking the base test harness. Unfortunately, it seems
to fail quite often, depending on testr ordering, and has been
blocking the gate quite a bit lately.

This patch refactors that test to be like all the rest of the
conductor tests. It also removes the useless mirror test of
test_vol_usage_update_again().

Fixes bug 1190971

Change-Id: I815b25f6585786a8dbd5eefcfb39e2b4790641cb
Merge "Fix overzealous conductor test for vol_usage_update"