Use RPC instead of neutron client in metadata agent

Use RPC instead of neutron client in metadata agent

RPC is a standard way of interacting between Neutron server and agents
Using neutron client is also  inefficient as it results in unneeded
keystone load and may become a bottleneck at scale

DocImpact
When upgrading, one should upgrade neutron server first,
then metadata agent. However there is a fallback in case
metadata agent fails to get info from server by rpc -
it will return to using neutron client.

Closes-Bug: #1092043

Change-Id: Ib75568643e6062543c0ff8cf80fba1b40b4fe82f