Refactor: extract method: driver_dict_from_config

Related to blueprint xenapi-volume-drivers

Extract method to create driver registries, so that it could be re-used
in xenapi for a similar purpose.

Change-Id: Ifd25d12335ac96a15dfea4c5ab7f315ff7ddfd8b
add config options for volume drivers

Change-Id: I0d029e77badfc63be77b4f8d93e3ef102034a262
move volume driver configuration to volumeops

Change-Id: I9484a7fb2987454d6c0aaf7aa690967cc38bbcf0
Refactor: rename to_driver_registry to driver_dict_from_config

Change-Id: I633c9b3e630f6e0e856328ea98d8a826a87805cc