Use a separate command for special plugins functionality

[thrix]

Currently tmt run has some functionality that is actually outside of the scope of running a plan.

As an example, tmt run provision -h minute --list-images lists images, but due to this implementation it also creates a run, which is kinda weird, does not feel clean. Also can introduce some additional problems due to this implementation choice.

I would propose to move this kind of functionality out to another command, something like

tmt plugin provision minute --list-images

I would like to omit the --how as we are not provisioning here anything ... but it is up for discussion

tmt plugin list

Could show all available plugins for all available steps maybe.

Ideas are more than welcome.

[lukaszachy]

IMO it can be tmt plugin [STEP] PLUGIN so if plugin has unique name it can be ran as tmt plugin minute (as well as longer tmt plugin provision minute) For listing I'd say tmt plugin --list is better (if we ever have plugin called list :)

This will be handy to have more standalone features for step plugins, e.g. #965 I can imagine that in the future there might be need for login/token configuration (e.g. similar to bugzilla login --api-key which creates local config and makes sure your provides api-key works. Run once and forget about it.)