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

(cherry-picked from 11f5e87a0fe9f9adefcc2b814c4d26950f7dd760)
Change-Id: Idc47b6bda3ea38e582501278f82c9c6841b8cdf0