Remove check for mongo 2.4 since this is the mongo_3_0 role
This was probably carried over from the mongo 2 role. I see no reason
why mongo 2.x and mongo 3 couldn't coexist on the same machine.

Remove old hugepages init script check
This was probably added while we were still iterating on our mongo 3
deployment, but it should no longer be necessary.

Clean up ansible syntax

Don't move the old mongo data dirs
This actually skips for edX because we provision machines that already
have {{mongo_data_dir}} mounted on an external disk.  However, for
non-edX use, this could fail if you turn on WiredTiger since it will
move mmapv1 files into the mongo_data_dir and then mongo will fail to
start because it has been told to use WiredTiger.

Don't make this a serial play
We usually run this on 3 machines, so serial: 3 was equivalent to
ansible's default of "run everything in parallel" but this causes
problems if you ever run it on 4 like we do for prod envs.
In addition, this prevents run_once from working properly, and there are
a number of things we only one to do on one machine (like creating a
superuser).