Add verification that TestVM image exists

Added method that verifies if TestVM instance exists.
Added method invocation to all tests that use default
image. Moved get_image_from_name method to
OfficialClientTest class.

Change-Id: I9404e312009b2f98d0a3904f347fe3c1e4a8fc1d
Closes-bug: #1282456
Merge "Add verification that TestVM image exists" into stable/4.1