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
replaced deprecated parameter notifier_strategy

The configuration option 'notifier_strategy' has been deprecated
in favor of 'notification_driver'.

Change-Id: Ia51aa6772df8736c57f0fb2223b6a680e3dfb11d
Merge "Introduce glance:config to manage custom options"
Merge "replaced deprecated parameter notifier_strategy"