ChrootManager: clean StopInstance

Currently it has lots for duplicated code, and internal retries.
Clean it up with the following assumptions:

We'll probably be called more than once.
It is ok to fail to stop, unless we're called with force=True.
If we're called only once, and with force=True it's ok not to run the
chroot "cleanup" script (it's a destroy after all, why should chroots
have more chances than other instances?).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>