Feature/suggest vlans dinamically

Suggestion of vlans and unit numbers is done from the vlan range and taking into account vlans already used in other VCPEs.

Suggestion is done in a way it does not repeat same vlan for same physical interface in the same VCPE. It does so for units, too.

In order to do that, we now keep track of already suggested vlans and units in current vcpe,
and suggest a particular one only if it has not been suggested already (in the same vcpe).

Suggestion also calls isFreeVLAN for a candidate vlan in order to check the vlan is not being used in other VCPEs.

Linked to issue:
http://jira.i2cat.net:8080/browse/OPENNAAS-833