RHEL6 package provide file /usr/bin/pip

On RHEL6, the pip-python package provide:
the /usr/bin/pip file
the /usr/bin/pip-python link (on the pip file)

The python::pip class set /usr/bin/pip as a link on /usr/bin/pip-python, then we have two dead links.

Maybe a previous package (for rhel5?) provided a file pip-python without pip link (?)

This patch ensure we don't destroy the pip file provided by the package, for RHEL >=6