fix #1378: allow to specify a service shutdown policy

fix #1378: allow to specify a service shutdown policy

Allow an admin to set a datacenter wide HA policy which can change
the way we handle services on a node shutdown.

There's:

* freeze: always freeze servivces, independent of the shutdown type
  (reboot, poweroff)
* failover: never freeze services, this means that a service will get
  recovered to another node if possible and if the current node does
  not comes back up in the grace period of 1 minute.
* default: this is the current behavior, freeze on reboot but do not
  freeze on poweroff

Add to tests, shutdown-policy1 which is based of the reboot1 test,
but enforces no freeze with a failover policy, and shutdown-policy2
which is based on the shutdown1 test but with a explicit freeze
policy. You can compare (diff) each tests log result to the test it's
based on to see what changes.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>