Adjust code to new pip 10
Adjust code to new pip 10

IR is unable to add plugins with plugin_requirements file in them
since pip 10 has no longer main attribute.

Adjusted code to pip 10 and fallback to older pip in case of exception.

RHOSINFRA-1721

Change-Id: Iec01a53d42ca2594b37d259e93dec4aeabb70d3f
(cherry picked from commit 6b2b5f8a8b25627317fc8bdbd468f156d5a8c556)

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared