L3VLAN capability unassign IP address

Implementation  of unassign IP address method trying to fix [OPENNAAS-1378](http://jira.i2cat.net/browse/OPENNAAS-1378).

**Throwing UnsupportedOperationException while bug not solved:**
http://forums.juniper.net/t5/Junos/quot-syntax-error-expecting-lt-rpc-gt-or-lt-rpc-gt-quot-trying/td-p/235936