Fix cirros MD5 fails

The awk which extracts MD5 for cirros images is not correctly expanding
the image id shell variable. There was also a superfluous }.

Additionally, if a previous build was done without USE_CIRROS then there
is a cache-invalidation error when downloading the new user.qow2.

Change-Id: I3def8518e6b05c83a8e2aa2a911aa1e66f23db85
Closes-Bug: #1317117
Merge "Fix cirros MD5 fails"