Don't run mongodb_instance in default recipe for replicaset.

Fixes #55. Prevents service restart because of unnecessary configuration change. Works when replicaset recipe is included from another recipe. Tested on Chef 10.14.2 (vagrant box http://files.vagrantup.com/precise64.box) and 11.4.0 (http://puppet-vagrant-boxes.puppetlabs.com/ubuntu-server-12042-x64-vbox4210.box).