Handle compute node not available for live migration

This patch handles exception.ComputeServiceUnavailable by restoring
instance's vm_state and instance's task_state after live migration
failure caused by unavailable source/dest compute node.

Raises detailed HTTPBadRequest explanation for this exception.

Fixes bug 973393 and bug 1051881

Change-Id: If825b61fad9c4e3030f2e6c5002907255eaf3661
Fix update quota with invalid value

If update quota with a value which is not integer, it will response
with http status code 500. Catch the exception and continue for
updating other quotas.

Fix LP# 1111327

Change-Id: I0ab3b10ff106a2e6c7677d1acdb55c2f35c492db
Forces flavorRef to be string in servers resize api

flavorid column is a string in the database model and
MySQL silently accepts either integer or string as
data type. However PostgreSQL expects the right data
type. So we force it to be string as early as possible
in the api.

Fixes Bug 1123525

Change-Id: Id117868b3a2f840f5d7ab463f3b79b6631b3b514
Merge "Forces flavorRef to be string in servers resize api"
Merge "Fix update quota with invalid value"
Merge "Handle compute node not available for live migration"