Prevent thrashing when deploying many bm instances

Nova BM is easily overwhelmed at the moment because it has to convert
every disk image to raw - simply deploying 2 instances at once exceeds
the IOPS on most single disk servers. Avoid this by serialising the
image conversion (which leads to a lower average-time-to-deploy in
nearly all situations).

Change-Id: I6d5ee97ffdbd497802379b13e70b84d70d8ded57
Co-Authored-By: Steve Kowalik <steven@wedontsleep.org>
Merge "Prevent thrashing when deploying many bm instances"