make neutron-ns-metadata-proxy configurable

Introduced a new configuration parameter for DHCP and L3 agent
to be able to set the path of the metadata socket to be used
by neutron-ns-metadata-proxy when started inside the DHCP or L3
agent. Also moved the configuration parameter of the
neutron-ns-metadata-proxy to the CLI parameters.

fixes bug #1202628

Change-Id: I3b1aa72b4e06c00f7fed152f0ee03d52b6bbc279
Merge "make neutron-ns-metadata-proxy configurable"