Ensure port is a string in psql command

In 841187b0ec29726c0fe39ea2ad710a5ead42d2e0 a shell execution
vulnerability was fixed by passing an array. In my environment (Debian
10, Ruby 2.5, Puppet 5) it was failing because port was an Integer and
command execution only allows Strings. This explicitly converts the port
to a string.

Fixes: 841187b0ec29726c0fe39ea2ad710a5ead42d2e0