Use RPC instead of neutron client in metadata agent

Previously, neutron metadata agent connected to a neutron server via REST 
API using a neutron client. This is ineffective because keystone is then 
fully involved into the authentication process and gets overloaded. The 
neutron metadata agent has been reworked to use RPC by default to connect 
to a server since Kilo release.

Change-Id: Idaf9fbdb33ad3d5f68f40eef54344bffd842f0a0
Closes-Bug: #1386979
Merge "Use RPC instead of neutron client in metadata agent"