Fix resize when instance has no image

This patch makes it possible to resize an instance that has no image
supplied (was booted from volume), by avoiding calling the image service
and thus causing an uncaught exception, if there was no image suplied in
the first place.

Fixes bug: 1182114
blueprint: improve-boot-from-volume

Change-Id: I67d63a7aef699b555897582c74b04e857db7aafb
(cherry picked from commit 4465b705fc7cb7a150a7f8d6f70ccf6c20ff3284)
Merge "Fix resize when instance has no image" into stable/grizzly