L3 Agent should generate ns_name in a single place

L3 Agent should generate ns_name in a single place

Currently the l3 agent has 2 places where it allows generating ns_name
of specific router_ids (ie. qrouter-<router_id>): in the RouterInfo's
constructor, and in _cleanup_namespaces. This patch proposes a
unification of this creation code with a property which lives in
RouterInfo's namespace. A simpler fix was also made for snat_ns_name.

This patch also offers a single way to initialize a new RouterInfo.

Related-bug: #1374946
Related-bug: #1374947
Change-Id: Ia028236b73a22ff534acee00b46c86b134dc987e