Fix openstack server provider resource cleanup

* Pass the correct parameter to ResourceManager.delete (integer id, not
  resource object)
* Wrap each removal (os api and rally db) in individual try-catch, so
  that even if one resource has already been deleted (eg instance
  deleted manually) then all the other related resources (eg keypairs)
  will still be cleaned up.
* Expand fakes.FakeResource.
* Update tests

Change-Id: If9ce3d20493248779d0ea3708470847eee53dc72
Closes-Bug: #1277348