Convert instance reinstall to multi instance model

This patch converts ‘gnt-instance reinstall’ from single-instance to
multi-instance model; since this is dangerours, it's required to pass
“--force --force-multiple” to skip the confirmation.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>