Rework the custom plugins builds hack to make it easier to add/remove a plugin

Currently, we have a custom build for Essentials for the configuration-as-code plugin, and the essentials one.

I am about to:

• remove essentials once HOSTING-541 is processed and we can release it
• remove configuration-as-code as we now switch back to the latest release since we do not use anything specific in the master branch
• add the metrics plugin to move forward as long as https://github.com/jenkinsci/metrics-plugin/pull/30 is not released (it is already merged at the time of writing)

We can expect adding/removing plugins is going to be pretty common in the next weeks as we move forward. So we should have simpler way to do that instead of basically copy-pasting or removing some lines of Makefile around to add or remove a new plugin, and commenting out or not the line in the plugins.aria file currently...