Install python-jenkins on Salt master

Summary:
This is required by the Jenkins execution module.

Reference: https://docs.saltstack.com/en/latest/ref/modules/all/salt.modules.jenkinsmod.html

Ref T1458

Test Plan: salt-call --local state.sls roles/saltmaster/software

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1458

Differential Revision: https://devcentral.nasqueron.org/D1846