Install Guide: configure the neutron metadata agent

VMs won't be able to access the metadata API if the Neutron metadata
agent is not configured.

closes-bug: #1252521
closes-bug: #1252522
backport: havana
Change-Id: I531f04a2a972b7f9e5ddc1508afac752c01ab563
Merge "Install Guide: configure the neutron metadata agent"