do simple fallback if node comes back online from maintenance

do simple fallback if node comes back online from maintenance

We simply remember the node we where on, if moved for maintenance.
This record gets dropped once we move to _any_ other node, be it:
* our previous node, as it came back from maintenance
* another node due to manual migration, group priority changes or
  fencing

The first point is handled explicitly by this patch. In the select
service node we check for and old fallback node, if that one is found
in a online node list with top priority we _always_ move to it - even
if there's no real reason for a move.

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