When pip is run outside of venv, don't show the upgrade warning
This is a fix for https://bugzilla.redhat.com/show_bug.cgi?id=1573755
1. We put "rpm" inside pip's INSTALLER instead of "pip"
2. From pip, we check what's in INSTALLER and only show the warning
if it's "pip".
When a venv is cearted, pip is installed from wheel (trough rewheel),
INSTALLER contains "pip".
When virtualenv is used, pip is installed from the Interwebz via pip,
so INSTALLER contains "pip".
Upstream issue https://github.com/pypa/pip/issues/5346