Add models module, for objects we pass to plugins

Move EntitlementSource to models.

Make EntitlementDirEntitlementSource a container
of models.Entitlement's (or models.EntitlementCertEntitlement
for our case).

EntitlementSource is an abstract version of EntitlementDirectory,
and Entitlement is a abstract version of a EntitlementCertificate.
EntitledContents is a abstract version of
EntitlementCertificate.content (a list of rhsm.certificate2.Content
 currently).

Add test cases for models.
Remove 'api' module.

ostree_content plugin now imports base_plugin.
stylish cleanups
Add a static map_gpg to OstreeRemote

Used to map certificate2.Content.gpg to a remotes
gpg-verify value.
Make models internal data attrs '_'