Suggest VRRP group dynamically.

Preferred VRRP group is down interfaces vlan tag. Selected from DOWN1 interface.
If preferred VRRP group is already assigned to another VCPE,
then suggested group is the first free VRRP group identifier.
Launch exception when there is no valid suggestion.
Merge pull request #500 from isartcanyameres/feature/vcpe-suggest-vrrp-dynamically

Feature/vcpe suggest vrrp dynamically