Make omni_db_createusers.psql callable from automation scripts

* Remove hardcoded connect to database named ‘omniwallet’
* Add ‘drop role if exists’ before creating users
* Add PSQL variables for passwords which must be set externally
* Update scripts/db-init.sh to call omni_db_createusers.psql
The variable usernames were used everywhere else in this file except on this line. Good catch. I pushed another commit which fixes this.