Instead of clearing existing reservations at the beginning of
del_host, wait for the tasks holding them to go to completion. This
check continues indefinitely until the conductor process exits due to
one of:
- All reservations for this conductor are released
- CONF.graceful_shutdown_timeout has elapsed
- The process manager (systemd, kubernetes) sends SIGKILL after the
configured graceful period
Because the default values of [DEFAULT]graceful_shutdown_timeout and
[conductor]heartbeat_timeout are the same (60s) no other conductor
will claim a node as an orphan until this conductor exits.
Change-Id: Ib8db915746228cd87272740825aaaea1fdf953c7