Introduce neutron:config to manage neutron configuration

This neutron::config is aim to use all types of neutron
config resources to manage custom configurations in all
neutron config files.
This will make end user easy to add their own custom options
in Hiera data.

Fully implements blueprint neutron-custom-config

(cherry-picked from e420021699f5fd0b5b13cbf8b070070ba8e03b9b)
Change-Id: I2a6b79a48f011c84a8b73467c9bf813741f6e898
Merge "Introduce neutron:config to manage neutron configuration" into stable/havana