Introduce keystone:config

This keystone::config is aim to use keystone_config resources
to manage custom configurations in keystone config files.

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

Fully implements blueprint keystone-custom-config

(cherry-picked from 9b8801866e7eeeb330d426930286d5221ba1682f)
Change-Id: I1e0742382bfca3f627029c7f17fe05fd9d3d0dbe