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
Change-Id: Ic5eedb8057c7f4934eed08869ebf55c91e6edfc9
Merge "Metadata agent caches networks for routers"