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
right, but this line is the create user command which has static usernames, not the variables.  
create user omniengine ..  vs   create user :omniengine 
right?