Adds missing db registry api tests for Tasks

The db registry api supports calls to Glance tasks. This patch
adds test coverage to that part of the code.

Fixes bug 1364570

Change-Id: Id7a74239a54d9001c7ac4ab4c8e2b67ac0686125
Improve OS::Compute::HostCapabilities description

The OS::Compute::HostCapabilities namespace should include more
information about how the properties are used by Nova.
Let admin's know that the ComputeCapabilitiesFilter needs to
be enabled.

Change-Id: Ib3232e560969462d3973606e138a96d1a5beb7de
Closes-bug: 1367860
Block sqlalchemy-migrate 0.9.2

Breaks unit tests.

Change-Id: I347a20fd35ae20f1de5db7f2608090b08af6c0ff
Closes-Bug: #1368391
Merge "Adds missing db registry api tests for Tasks"
Merge "Improve OS::Compute::HostCapabilities description"