Allow use of virtualenv via stankevich/python

 - This new pull request allows pip package install
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.

Detail 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