allow setting gunicorn's --worker arg

allows you to set the --worker argument.     

I would suggest the default actually be changed from 2,  but this is a least change PR. 

The problem is when using gunicorn sync worker type,  the default,   they can only handle one outstanding request at a time,  however in a cluster graphite-web makes blocking calls itself to other graphite webs  so it's possible to get the right set of queries that cause all workers across your cluster to be blocked on  responses from the other graphite-web server which are in turn not servicing their queue from nginx because they are blocked. 


graphite-web makes http calls to the other graphite web