If running git clean after reset then retry

* devstack-vm-gate-wrap.sh: If a git clean can not remove some
files, for example because a previous command has not freed its open
file handles, it will return nonzero. If this happens, wait a moment
and try again in case this is only a briefly transient problem.

Change-Id: I40ca9784f5e6e8bad07416f4fd1978bc7aecd494