Refs #7733: Remove usage of Host fixtures in favor of Host factories.

Foreman is moving to the use of factories for testing and removing
the host fixtures from usage. Here we change out our reliance on
those host fixtures in favor of the factories.
Fixes #7688: add a content hosts tab to the errata details.

Adds a content host tab to the errata details page that allows
the listing and syncing of applicable content hosts.

http://projects.theforeman.org/issues/7688