Replace mongodb_user module with javascripts

This add fullsupport of new authentication model in mongodb 2.6.

I know this isn't the most elegant fix, but using the latest module in ansible didn't work, and I was having issues modifying to work properly so I've fallen back to stripping that out and just using mongo javascripts.

I have tested this with the cluster build and devstack (including a "from scratch" on mongodb 2.6 devstack as well as an upgraded version.

One of the upshots is that it will actually work on a cluster and not fail on the slaves since the scripts check if they are master before attempting to modify anything so I removed the "create_users" flag and just run that each time.