restapi: Generic external provider

This patch adds a generic external provider class, to be used by the existing
and future external provider classes.

Change-Id: I040b0edc6d734337d238db4e90c3167fad0e39e8
Related-To:  https://bugzilla.redhat.com/1132259
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>
restapi: OpenStack external provider

This patch adds to the XML schema a new type for OpenStack external
providers. It only contains the tenant name.

Change-Id: I4210a48d5213d6369644881878fa1a41fcbe6217
Related-To: https://bugzilla.redhat.com/1132259
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>