Set /tmp as default CWD for postgresql_psql

This has been discussed in issue #148. The postgresql_psql type doesn't
change the CWD to something safe when running psql as the postgres user.
During a regular puppet run the CWD remains "/root", to which the
postgres user usually does not have access, resulting in failed psql
calls and failed puppet runs. This simple change makes "/tmp" the
default CWD for postgresql_psql.