Support Content Overrides through Hosts Bulk Options or Host Subscription

[lvw5264]

There are only three ways to set Content Overrides per host, of which only content overrides in activation keys is currently implemented in Nailgun.

1. Activation Keys (Implemented) Using activation keys to set content overrides as hammer and nailgun already supports is not really a good option since not all content overrides are supposed to apply to every host in an activation key (such as disabling certain content). https://github.com/SatelliteQE/nailgun/blob/master/nailgun/entities.py#L3617
2. Single Host In the Satellite 6.4 API, it supports setting content overrides through Host Subscriptions, but nailgun does not yet support setting this, so it needs to be implemented.
3. Multiple Hosts - In the Satellite 6.4 API, it supports Hosts Bulk Options to provide various useful bulk actions (vs having to send a bunch of API calls per host). One of these is already implemented, installing content to hosts in bulk. The API endpoint PUT /api/hosts/bulk/content_overrides is only exposed through Hosts Bulk Options, and cannot easily be accessed elsewhere.

Will it be possible to add single host content overrides to HostSubscription, or multiple host content overrides to Host using Hosts Bulk Options?