Updates to params.pp for mongo and postgres packages

*) Removed the hardcoded postgres package from db.pp
*) Changed the mongo package to python-pymongo for RHEL

Change-Id: Ib975f9ce03ee8ffa423e75b62b595bcdefdd1eed
Signed-off-by: Francesco Vollero <fvollero@redhat.com>
(cherry picked from commit 0fb3b8f693ba4cc24bdcfb1f61de843343a13e75)
Merge "Updates to params.pp for mongo and postgres packages" into stable/havana