Fix Python 2 package prefix for CentOS and RHEL servers

Summary:
Python 2 packages use a different prefix on Fedora and RHEL/CentOS:

• python2- for Fedora, currently defined in the map
• python- for RHEL/CentOS, fixed by this change

This is a follow-up for change 690ca84c957b.

Test Plan: no op

Reviewers: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1408