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
We need BASH for [[
Make :omniengine & :omniwww usernames variables
Add ‘:’ prefix in two places that I missed.
Merge pull request #26 from msgilligan/msgilligan-vagrant

Make omni_db_createusers.psql callable from automation scripts