Postgres: unify psql command execution in setup-script

[shivaraj-bh]

@adrian-gierakowski I think it would be nicer if there is one function that executes psql commands so that we don't accidentally add a psql command in the future that doesn't use psqlUserArg. That can be done in a different PR, merging this.

Originally posted by @shivaraj-bh in #106 (comment)

[adrian-gierakowski]

Sure, should it be a psql wrapper, or simply a bash snippet we can interpolate into the script?

[shivaraj-bh]

or simply a bash snippet we can interpolate into the script

This sounds good!