Ubuntu 18.04 LTS install

[viktorsperl]

This is almost identical to Debian 9: only pgdg repo distribution is different.
This PR also adds:

• Processes the LSB of Ubuntu (auto detects distribution)
• Option to set custom (Postgres) database password via ENV vars
• Option to set port numbers to various services via ENV vars (no need to edit script header)
• Fixes the database user grant, which was static previously

[CLAassistant]

All committers have signed the CLA.

[lmangani]

Major Thanks @viktorsperl for this useful contribution! We're about to switch the installer to the 7.7 repositories so I'm just keeping this on hold to correct/merge at the perfect time.