Add configuration support for Mattermost to use MySQL

Mattermost will work with MySQL database, it seems. Omnibus needs some configuration to allow this configuration, though. Please add this configuration for future GitLab omnibus packages.

Milestone changed to 8.0

Reassigned to @marin

mentioned in merge request !460 (merged)

!460 (merged) is merged, in the next package version you will be able to specify driver other than postgresql with: mattermost['sql_driver_name'] = 'mysql'

Status changed to closed

mentioned in issue #818 (closed)