Change RouterGateway resource's name

This patch changes RouterGateway resource's name building
logic and removes unused network_id from resource's name.
If name looked like 'router_id:network_id' earlier, now resource's
name looks like 'router_id'. Also, it still supports compatibility
with old resource's names, which look like 'router_id:network_id'.

Change-Id: I33b589870f26761802de84046281b0c626a9e03c
Merge "Change RouterGateway resource's name"