Add CONFIG_SCHEMA to openstack provider

SCHEMA_TEMPLATE was renamed to CONFIG_SCHEMA and moved to the class
scope.

Also, config validation was deleted from OpenStackProvider constructor
for using specified validation method in the base class.

Change-Id: Ia71d1858bd5a8b313e52188bfec3d95f100f5a8c
Related: blueprint common-config-validation
Merge "Add CONFIG_SCHEMA to openstack provider"