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
Fix instance obj refresh()

The refresh() method pulls a fresh copy of the instance but if any
fields changed when doing so, they are left marked as changes that
are not yet committed.

Also clarified the doc string for obj_what_changed() to state that a set
is returned, not a list.

This method is not used yet, so I didn't file a bug for this.

Change-Id: I83fbce07936fea2bc936441c5c6594f65da443a7
Imported Translations from Transifex

Change-Id: Iba2363891c83dda503518169617183930357ec80
Merge "Fix overzealous conductor test for vol_usage_update"
Merge "Fix instance obj refresh()"
Merge "Imported Translations from Transifex"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp