Allow use of virtualenv via stankevich/python

This new pull request allows pip package installs
at the system level or inside a virtualenv

 - Setting $python_provider to 'system' is the
default and shouldn't cause any deviation in behavior
 - Setting $python_provider to 'virtualenv' sets up
the /opt/graphite directory as a venv
 - If using a virtualenv, some other things need to be
aware of the virtualenv.

Details on the files that changed:

manifests/init.pp

 - add $python_provider variable

manifests/params.pp

 - add $virtualenv variable
 - remove OS packages that can be pip installed
 - this provided less EPEL dependencies

manifests/config.pp

 - setup $venv_environemnt variable to invoke venv

manifests/config_apache.pp

 - setup the $virtualenv variable to use in wsgi

templates/{RedHat/Debian}/*.pp
 - setup init scripts to use $venv_environment
 - this allows venv pip packages to be imported