New MongoDB formula, rewritten to use official packages and APT repo

Mostly a rewrite to use the official mongodb packages so we can use 2.6.5.

Also have tidied up and automated (tho still needs work) the admin user creation, and normal user creation.

There are helpers for replica set creation, and reindexing, recovery.

Is it worth documenting that we can't have `'` in passwords? (There's a non-zero chance we would since the passwords would be auotgenerated.)