Support for gn_db env variable added

[Zalitoar]

If gn_db isn't defined when run command is executed 'geonetwork' is used as default database name. 'db_admin' variable was deleted, it isn't used by the application.

[grmpfhmbl]

Quick question as I was about to implement the same change (and this PR is already over a year old): Why are you not using the same "default value" syntax as is already used for other variables e.g.

db_host="${POSTGRES_DB_HOST:-postgres}"

I'd suggest something like:

db_gn="${POSTGRES_DB_GN:-geonetwork}"