Add missing priority input to sprintf.
Make the witness DB have unix domain socket and specify the data dir.

The data dir is needed for use with the normal ubuntu postgresql setup.
Don't ask for a password in pg_ctl init so we can automate this from scripts.
Create a repmgr user and db on witness create.

This is currently using hard-coded values, should be from config.

I find I need to create the user and db before trying to connect to the
witness using the conninfo from the config file. Without it just fails
with no such db/user. One alternative would be to have conninfo
temporarily connect to postgresql db using postgres user, but that seems
awkward and means need to stick repmgr stuff into the postgres db
directly which seems wrong.