Metadata agent caches networks for routers

During cloud-init there are several calls that asks neutron API for the
same data which will not be most likely changed. Specifically router's
networks are cached.

Closes-bug: #1276440

Conflicts:
	neutron/tests/unit/test_metadata_agent.py

Change-Id: Ic5eedb8057c7f4934eed08869ebf55c91e6edfc9
(cherry picked from commit 3faea81c6029033c85cefd6e98d7a3e719e858f5)
Merge "Metadata agent caches networks for routers" into stable/icehouse