Use state transition checker wait function in the ec2 image tests

Now the test_register_get_deregister_ami_image can use the usual
transition checker wait logic.
Previously the bug 1074904 prevented it.

Change-Id: I75d90c92aaa50dc62f15a962a5f14ba395b2948c
Merge "Use state transition checker wait function in the ec2 image tests"