CLOUDSTACK-4786: Reset Redundant Router priority after all the routers are stopped

This patch would reset the priority in such condition:
1. All redundant routers are stopped, e.g. due to network GC
2. User start one VM in the network
3. The routers would be brought up with reseted priority(100 & 99).

This would resolve the issue of network GC result in lower limit of redundant router priority reached.
CLOUDSTACK-2792: Call savepassword.sh inside VR for Xen

Also only set password when password service is running, thus avoid setting for
redundant router BACKUP router.