(#15980) The pip provider doesn't work correctly on redhat based systems (ignore pip from easy_install).

Redhat may use pip-python from epel or pip from easy_install. Without this patch
the pip provider require pip-python command and ignore pip command installed by easy_install.
This patch fixes the problem and accept pip command from easy_install as a solution on redhat based systems.
Moreover, pip installed by easy_install is promoted when both version are installed.