Add Quantum metadata proxy documentation

Document metadata_agent.ini - includes auth_* and admin_* flags needed
for the Quantum client as the metadata agent does not seem to be able
to read these from quantum.conf even though it is on the command line.
Removed section in advanced config options about the need for
additional routing as this no longer applies.

Fixes bug 1099573

Cherry-picked from https://review.openstack.org/34576

Change-Id: I2f5b73d5a6bd2add27f9dd46ad2ca1d6db832b1c