Move entitlement dir entitlement source closer to it's parent class.
Drop models ContentSet and EntCertEntitledContentSet.

Essentially just a list, we can make things simpler by just using it as such.
Move code to find content by type somewhere it's reusable.

This operation is useful for the upcoming docker content plugin, likely any
content plugin. Moving up to EntitlementSource means we can eliminate another
object in the OSTree plugin.
Correct a small test error hidden by mocking.

The call we're testing actually expects EntCertEntitledContent objects here,
but we were mocking something to look like a rhsm.certificate2.Content. Mock
was hiding the error that a logger statement would have exposed.
Refactor EntCertEntitledContent.

This was a source of confusion when trying to write a content plugin as it's
very difficult to tell what object you're getting where, content is referenced
all over the place. In particular, this object carried a content, which callers
frequently referred to as content.content.

After some discussion, refactoring such that we leave rhsm.certificate2.Content
as a cert specific class, and use this as a more general class.

Removed the reference to the content object we're trying to hide, and copied
the data we use off it onto the new object. It continues to carry the
additional reference to it's cert.
Refactor generic model into it's own namespace.

Moved entitlement cert specific subclasses to their own module.
More generic search for content method on entitlment source.

We might someday want to scan for content by other criteria, potentially
combined.
Move find content method off entitlement source.
Merge pull request #1024 from candlepin/dgoodwin/content-plugin-refactor

Content Plugin Refactoring