Add Alias Support to mongodb collector

The memcache collector has a nice option with hosts lists where you can specify an alias name for each listed host.

eg.
hosts=cluster1@localhost:8000,cluster2@localhost:9000

I've used that a as a model to add similar support to mongodb collector.

(you might want to consider making 'hosts' a higher level primitive with generalized support at a higher level, but that's a much bigger task potentially hitting LOTS of collector code)

Tested in my environment.

Cheers,

Joel Krauska