scripts: Add ability to install a different pip version

scripts: Add ability to install a different pip version

@guits was seeing an issue in ceph-ansible jobs with pip 10.0.0.  This commit will allow individual jobs to specify "latest" or "pip==X.X.X" or "pip<20.0.0" as a second parameter to the `install_python_packages` functions.  The default behavior is still no second parameter and pip=10.0.0 will still be installed until we're ready to unpin that.

Signed-off-by: David Galloway <dgallowa@redhat.com>