Don't fix disks before rexecing

Do _even less_ before the rexec: the only action done is to
update $WORKSPACE/devstack-gate to the Zuul ref before doing
the re-exec (into the just-updated copy).

Change-Id: I74cb58ba35c4eeee6786a30ac35263758856f6f9
Merge "Don't fix disks before rexecing"