Move xnat pgsql database to a separate volume to decouple the application and its state

After completion of #39 (closed) we would like to create a separate volume for the xnat database. It would allow us to decouple the application and state. It also simplifies our deployment process.

Currently, the application state is maintained by the PostgreSQL DB. It's data directory /var/lib/pgsql is located on the root volume of the instance. We want the data dir to be on a separate volume that is mounted at /var/lib/pgsql.

For reference we can follow the steps previous done in this project https://code.rc.uab.edu/rc/terraform-openstack/-/blob/master/.gitlab-ci.yml?ref_type=heads#L48