Create external DB settings.

[skoulouzis]

If we want to use an external DB ig. PostgreSQL we need to set it in production settings

[gpelouze]

• [x] Merge https://github.com/QCDIS/VREPaaS/tree/62-create-external-db-settings into https://github.com/QCDIS/VREPaaS/tree/20-vre-catalog-prototype
• [x] Modify django production settings to use PostGIS instead of SQLite
• [x] Any other modifications? (Excluding files in k8s/ as these will be part of the Helm Charts)

[gpelouze]

Closed by https://github.com/QCDIS/VREPaaS/pull/49/commits/183cc0fd132374cca7fab83a9fb3907ef64f46bc