Use base.create_image_from_server to create server snapshot

In I8ee2e18925e7f4f09d10d857fb25f3d9b8e8bd42, SnapshotNotFoundException
is added to base.create_image_from_server to get more friendly
error info, so testcases should use base.create_image_from_server
as far as possible, moreover, the code will look simpler.

Change-Id: I34ae75d56b4243d67a1a9ec48d033b1e7d6356a7
4 files changed