Fix Trusted Filter to work with Mt. Wilson `vtime`

The Trusted Filter expects that `vtime` is returned from the
attestation server in an ISO8601 format, but the Mt. Wilson
attestation server can return it in a locale appropriate string format
instead. There's no way to configure Mt.  Wilson to do the right thing
so we should just try to parse the returned `vtime` as a string
formatted date.

Closes-Bug: #1353029

Change-Id: Ic7351e0463c014321bdb4fcfeba90ac51460b325
Use flavor in confirm-resize to drop claim

Current the confirm_resize() clean the stashed instance_type information
before invoking the drop_resize_claim, and the drop_resize_claim() try to
fetch the old instance_type in system metadata, which will be wrong.

We change the _cleanup_stored_instance_types() to pass back the
to_be_dropped instance_type, and use that information for drop_resize_claim.

Change-Id: Iba36acc75d3840104746596977b33bcd4e010f2e
Merge "Use flavor in confirm-resize to drop claim"
Merge "Fix Trusted Filter to work with Mt. Wilson `vtime`"