Introduce glance:config to manage custom options

This glance::config is aim to use glance_config resources
to manage custom configurations in glance config files.

This will make end user easy to add their own custom options
in Hiera data.

Fully implements blueprint glance-custom-config

Change-Id: Idc47b6bda3ea38e582501278f82c9c6841b8cdf0
Merge "Introduce glance:config to manage custom options"