Add support for Helga 1.7 plugins variables

[nrb]

Helga 1.7 added some new pllugins variables that would be useful to expose at the Ansble level.

These include:

• helga.settings.DISABLED_PLUGINS - blacklist of plugins not allowed to run (default is none; all plugins can run)
• helga.settings.ENABLED_PLUGINS - whitelist of plugins allowed to run (default is all plugins)
• helga.settings.DEFAULT_CHANNEL_PLUGINS - list of automatically enabled plugins in channels (old enabled_plugins value).

These are explained in depth here.

Also, most of the helga plugins originally included were separated out, so I think it makes sense to either add them as the default list of external plugins within the role, or to have some sort of default list that's auto-appended. A list of the removed plugins can be found in this commit

The goal would be to still include these, at the ansible level, as defaults, but update the machinery that makes that happen.