Clean environment before calling psql

If environment variables are not cleaned, the psql execution can be
affected by variables like PGHOST, and induce unwanted behaviours or
failures.  Especially on Linux if one launches salt-minion by invoking
directly the init script instead of using the "service" command (the
former does not clean the environment).
Merge pull request #12227 from tmartinfr/psql-clean-env

Clean environment before calling psql