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.

They are now requirements of the addition of the APT key for the mongo-org repo.

Basically, ensuring that the old packages have been purged -- they are *totally* incompatible with the mongo-org ones, so we need to be harsh.