Add a --list-plugins hook

[ekohl]

This adds an installer option --list-plugins hook. It is implemented in pre_validations because that allows showing whether they are enabled or not. This does mean that the verbose output is always shown prior to it.

Example output

$ RUBYOPT=-W0 be ./bin/foreman-installer --scenario foreman --list-plugins --skip-checks-i-know-better
2020-11-10 12:18:12 [NOTICE] [pre_migrations] Executing hooks in group pre_migrations
2020-11-10 12:18:12 [NOTICE] [pre_migrations] All hooks in group pre_migrations finished
2020-11-10 12:18:12 [NOTICE] [boot] Executing hooks in group boot
2020-11-10 12:18:12 [ERROR ] [root] Command foreman-maintain packages -h not found
2020-11-10 12:18:12 [NOTICE] [boot] All hooks in group boot finished
2020-11-10 12:18:12 [NOTICE] [init] Executing hooks in group init
2020-11-10 12:18:12 [NOTICE] [init] All hooks in group init finished
2020-11-10 12:18:12 [NOTICE] [root] Loading default values from puppet modules...
2020-11-10 12:18:15 [NOTICE] [root] ... finished
2020-11-10 12:18:15 [NOTICE] [pre_values] Executing hooks in group pre_values
2020-11-10 12:18:15 [NOTICE] [pre_values] All hooks in group pre_values finished
2020-11-10 12:18:15 [NOTICE] [pre_validations] Executing hooks in group pre_validations
* Foreman cli
  * ansible (disabled)
  * azure (disabled)
  * discovery (disabled)
  * kubevirt (disabled)
  * openscap (disabled)
  * remote_execution (disabled)
  * tasks (disabled)
  * templates (disabled)
* Foreman
  * ansible (disabled)
  * azure (disabled)
  * bootdisk (disabled)
  * chef (disabled)
  * column_view (disabled)
  * default_hostgroup (disabled)
  * dhcp_browser (disabled)
  * digitalocean (disabled)
  * discovery (disabled)
  * expire_hosts (disabled)
  * hooks (disabled)
  * host_extra_validator (disabled)
  * kubevirt (disabled)
  * leapp (disabled)
  * memcache (disabled)
  * monitoring (disabled)
  * omaha (disabled)
  * openscap (disabled)
  * ovirt_provision (disabled)
  * puppetdb (disabled)
  * remote_execution (disabled)
    * cockpit (disabled)
  * salt (disabled)
  * setup (disabled)
  * snapshot_management (disabled)
  * statistics (disabled)
  * tasks (disabled)
  * templates (disabled)
* Foreman compute
  * ec2 (disabled)
  * gce (disabled)
  * libvirt (disabled)
  * openstack (disabled)
  * ovirt (disabled)
  * vmware (disabled)
* Foreman proxy
  * ansible (disabled)
  * chef (disabled)
    * infoblox (disabled)
    * remote_isc (disabled)
  * discovery (disabled)
    * infoblox (disabled)
    * powerdns (disabled)
  * dynflow (disabled)
  * monitoring (disabled)
  * omaha (disabled)
  * openscap (disabled)
  * pulp (disabled)
    * ssh (disabled)
  * salt (disabled)

There is still a TODO which can mostly be seen in the Foreman proxy part, hence currently a draft.

[ekohl]

There aren't actually used and I couldn't quite get them to work in debugging. Perhaps they should be removed.