Idempotency issue with implicitly enabled plugins

[wyardley]

rabbitmq_pluginwas retrieving a list of existing plugins by querying for "explicitly" enabled plugins using therabbitmq-plugin -E` command. This caused us issues when adding support for CentOS/RHEL8 where we noticed that in this mode, ordering of plugins matters.

Example of the scenario we saw:

In this spec test: https:/voxpupuli/puppet-rabbitmq/blob/master/spec/acceptance/parameter_spec.rb#L18

The following plugins are declared:

rabbitmq_plugin { [ 'rabbitmq_federation_management', 'rabbitmq_federation' ]:
  ensure => present
}

The rabbitmq_federation_management plugin depends on rabbitmq_federation, so when our code enables rabbitmq_federation_management it will automatically enable rabbitmq_federation, but mark it as being "implicitly" enabled because it was enabled as a result of enabling rabbitmq_federation_management.

Then, when go to query for the list of enabled plugins using the -E option, this only lists our "explicitly" enabled plugins, so rabbitmq_federation is never listed and every time we run Puppet it attempts to enable the plugin, breaking idempotency.

Copied this text from @nmaludy's #844, but creating an issue as well, just for tracking purposes.

This also implements a different fix for what was originally fixed in #909