compute/images/test_images_oneserver: don't share resources

I think we can kill some code and reduce complexity at the
small price of creating one more server. (i.e not reusing/sharing
servers between tests). This way we can delete
the _rebuild_server_when_fails method which is ugly and fragile

Change-Id: I8956599a60bf78899b23896a70a0644ae7df0a2e
1 file changed