restapi: Provider certificate resources

This patch adds new resources intended to handle the digital
certificates of external providers. This resources will be used later
when introducing the resources for the external providers themselves.

Change-Id: Ibde73089c800138845fb2bc5717853f914aa0389
Related-To: https://bugzilla.redhat.com/1132259
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>