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
Merge "Fix resize when instance has no image"