Fixes #6055 - HC add/remove-content-host take uuid

the host-collection command's --system-id option was accepting database
ids instead of uuids. This uses the
add_host_collections/remove_host_collections action instead of the
AssociatingCommands (which uses the update action). This is a simpler
approach than trying to alter/override the behavior of an
AssociatingCommand to accept uuids.
Merge pull request #180 from dustint-rh/hc_rm_associating_commands

Fixes #6055 - HC add/remove-content-host take uuid