Postgresql is an Optional dependency

psycopg2 is for supporting sqlalchemy connection to
postgresql databases. The tempest testing jobs are allowed to use
the system python librairies so you just need to install your
favourite sql driver.

The mysql driver already removed from the requirements.txt.
This change removes the psql driver dependency.

Change-Id: I8eb972cf404c2a86c8acdb656fc1062fca293299
Merge "Postgresql is an Optional dependency"