Support cinderclient http retries.

HTTP retries were recently added to cinderclient. This adds the
config option required to use it. The default (unchanged) behavior
is 0 retries.

Change-Id: Ia7ad831045e8df9ad9c22f0119ae3485d91ad949
Add exception handler for previous deleted flavor.

An exception happens if a previous flavor is deleted and
'nova resize-revert' run, because 'nova resize-revert' does
not handle the deleted flavor.
And also we have the same problem in _update_usage_from_migration().
This patch fixes the problems.

How to reproduce the problem on DevStack:
  $ nova flavor-create sample 10 512 0 2
  $ nova boot --image cirros-0.3.0-x86_64-uec --flavor sample test01
  $ nova resize test01 m1.tiny
  $ nova flavor-delete 10
  $ nova resize-revert test01

Before applying this patch:
  $ nova resize-revert test01
  ERROR: The server could not comply with the request since it is
  either malformed or otherwise incorrect. (HTTP 400) (Request-ID:
  req-b0d3e016-9608-4a87-a0cc-44dfe00b25a1)
  $

After applying this patch:
  $ nova resize-revert test01
  ERROR: Flavor used by the instance could not be found. (HTTP 400)
  (Request-ID: req-ed4ce174-33f2-4258-b522-674a1023ea74)
  $

Fixes bug 1091490

Change-Id: I39dd23a7565ae66544e8bc2aa7ad3299eb61bfcc
Merge "Support cinderclient http retries."
Merge "Add exception handler for previous deleted flavor."