Removed duplicated installation of python psycopg2

It turned out that the python psycopg2 package is already
installed as part of this role and so its addition when it was
moved from the ekahau playbook was superflous.

I have removed the installation but I have added a cache option
to the first installation to ensure that the cache is updated for
all installations in this role.

Issue: IWSoftware/ekahau-rtls-playbook#1
Ticket: #5507

Signed-off-by: Nathan Callister <ncallister@iwsoftware.com.au>
Merge branch 'master' of ssh://github/IWSoftware/postgresql-role