Allow deleting instances while uuid lock is held

Avoid issues where locked up operations on the instance prevent it
from being deleted by the user. Reasons for the lock up are a separate
issue and will be handled elsewhere, but terminating the instance
should not affect any tasks anyway. Any modification should already
gracefully handle the instance going away.

For more discussion about the issue see:
http://lists.openstack.org/pipermail/openstack-dev/2013-October/017454.html

Partial-Bug: #1248563

Change-Id: Ife26ffefccea11d6e9ca5fbd970192b322fc04fd
disk/api.py: refactors extends and adds missing tests

The method extend does not log any information if there is a
problem during the resize with "resize2fs".

This patch refactor extend and adds missing tests.

Change-Id: I81079a79e0449381440f202f6143b48f96453a11
Related-Bug: #1270086
Merge "Allow deleting instances while uuid lock is held"
Merge "disk/api.py: refactors extends and adds missing tests"