When running Ceilometer in WSGI, 'httpd' service will need to be allowed
to write in /var/log/ceilometer.
Instead of hardcoding 'swift' user for being part of 'ceilometer' group,
create a new parameter 'group' so we can override which user we want
part of 'ceilometer' group.
Change-Id: Ib4c63ea386e93161cddb44c0a588f8e658c214e4