take two at gr_web_server = none, renamed to wsgionly plus bugfixes

this now allows the installation of gunicorn standalone, allowing me to use my own WSGI capable webserver configured externally from this module (eg. using the puppetlabs nginx module). the last attempt at this which was already merged only did half the job. I had to split the nginx and gunicorn logic to make this happen, when nginx is specified the logic for both is executed.

also fixed an issue where permissions for /opt/graphite/storage/whisper and /opt/graphite/storage/log/carbon-cache had incorrect ownership preventing logs and metrics being written if gr_user was specified.