Omnibus upgrade removes Mattermost extra installed plugins

Summary

Every Omnibus gitlab-ce upgrade, removes extra installed Mattermost plugins

Steps to reproduce

1. Install extra plugin in Mattermost
2. Upgrade Omnibus gitlab-ce

What is the current bug behavior?

• The installed plugin is removed, needs to be installed again

What is the expected correct behavior?

• Installed plugin must remain installed

Configuration details

It's been happening on every major/minor Omnibus upgrade, latest test done with gitlab-ce_13.1.1-ce and Mattermost plugin for BigBlueButton