Ensure pip is installed in order to invoke it

Summary:
This is a follow-up for dd7aee0e.

Ref T1595.

Test Plan: salt-call state.sls roles/paas-docker/salt

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1595

Differential Revision: https://devcentral.nasqueron.org/D2214