Katello: System Group to Host Collection rename changes merged to master

The changes to Katello to rename 'system groups' to 'host collections'
throughout the engine (schema, routes, UI, CLI…) have been merged to
master.

In order to access them,
- UI: Hosts -> Host Collections
- CLI: 'host-collection'

Developers,

The git repos with these changes include katello & hammer-cli-katello.

Once you update your katello branch based on master, be sure to run the
following to get your environment in sync:
rake db:migrate
rake katello:reindex

Thanks,
Brad