core: Add provider additional properties

The additional properties can change by provider type, so adding them as
a field in the Provider entity.
The first type of additional properties will be the
OpenstackNetworkProviderProperties which add a tenant name and plugin
type that are specific to the OpenStack Network provider.

Change-Id: Id9b51387a1bb7e4fb520ba53428ae82d9705e809
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>