Early die if on image gets killed

The Images reaches the 'killed' status only on an
unrecoverable error, any further wait is in vain.

The wait method will raise an exception on the killed state.

Change-Id: I392674c872d7f2354a2a2d338e14630b211cf12b
Replace assertLessEqual - is not in py26 testtools

Fixes bug 1233513.

Change-Id: I22ba767739aa79fd6fdf75167b2573a27625e6d1
Merge "Replace assertLessEqual - is not in py26 testtools"
Merge "Early die if on image gets killed"