set default mongodb version to 2.4.6, fix to :extra_opts handling (strin...

set default mongodb version to 2.4.6, fix to :extra_opts handling (string, not array)

Fairly straightforward version bump to 2.4.6 and fixing the :extra_opts template var.

I tested booting 3 node replica sets on both stable-v3 and stable-v4 with these changes in place.