Fix argument name mismatch in L3-RPC sync_routers

Fix argument name mismatch in L3-RPC sync_routers

In sync_routers L3-RPC method l3-agent sends router_ids but the
server side expected router_id. This commit fixes the server side
to accept router_ids, and drops "fullsync" arg from the agent side
(fullsync is not used anywhere and it does not affect RPC signature).
This change allows l3-agent to sync only the specified routers
instead of all routers.

Fixes bug #1201553

As a result of the above change, auto_schedule_routers() and
list_active_sync_routers_on_active_l3_agent() in L3 scheduler
needs to handle a list of router IDs. This commit changes L3 scheduler
to accept a list of router IDs in the above two methods.

Also fixes the argument order of fullsync and router_ids in get_routers
in L3PluginApi. L3-agent main code expects router_ids as the second arg.

Change-Id: I22e8d11b9676cbcfe9e72449031bb63071be8314