always push down metadata rules for router, not just if gateway exists

always push down metadata rules for router, not just if gateway exists

bug 1051842

the l3-agent only pushes down some of the DNAT rules for metadata mapping
if there is a gateway set on a router. However, the nova-api server could
itself be running in the local router namespace (or on the same box as
the quantum-l3-agent in the case were namespaces are disabled. In fact,
with namespaces disabled, this is the likely setup for a simple
environment). Thus, we should simply always push down the metadata nat
rules for a router, router than waiting until a gateway is added.

This patch also refactors the actions taken by the l3-agent when a router
is added or removed into a separate function for improved readability,
as add/removing the nat rules made these code segments even longer.

Change-Id: I3c6eb35b51df3babf747dbcff7f943b850e69838