The command will now default to the image currently in-use by
the server, effectively making the --image parameter optional.
This commit also adds basic tests for ServerRebuild since there
wasn't any. Will add more full tests for it.
Change-Id: I733fd3ad5a825f06563c72aa430122e1a0e3b3b0
Closes-bug: #1524406
Co-Authored-By: David Moreau Simard <dms@redhat.com>
Co-Authored-By: Tang Chen <tangchen@cn.fujitsu.com>