Configurable redis key

Change to allow the user to specify their own redis key. This lets the user to change the way front ends are mapped to backends.

E.g. in my scenario I have a cluster of machines. On each of the machines there is a host on port x that has a hipache reverse proxy in front of it. What I need to to is to be able to change the routing per machine even when using a redis cluster.

While it would be possible to use the redisDatabase config option, it does not work well when you have many nodes in a cluster or many such apps to configure.

This change lets me override the default behavior e.g. to use the machine's name (os.hostname()) as the redis key. That way traffic can be configured per server even with a shared redis cluser.

Thanks