Refactor Nova server resizing to be self contained

The split between calling server.resize() and check_resize() makes
it more complicated to add other tasks that can be done in parallel
with other servers' tasks. By making a resize method which is self
contained we simplify the addition of rebuild in a subsequent patch.

Change-Id: I0490476dbef30571e90b88163fc8ca0670458682
Merge "Refactor Nova server resizing to be self contained"