omnibus install doesn't pull in postgresql package

[MynaITLabs]

While following the https://docs.mattermost.com/install/installing-mattermost-omnibus.html instructions on installing Omnibus. The packages pulled with mattermost doesn't include postgresql auto-magically.

I had to run '$ apt install postgresql' manually to get the package.

BACKGROUND: We have MariaDBl installed with previous MM source code installation. After finding out that postgresql is required, we backed away from Omnibus installation/upgrade process. Discovered that upgrade path is not clean from MariaDB.)