Allow downloadable cartridges to appear in rhc cartridge list

Implements: Trello card origin_broker_184

An administrator can import a downloadable cartridge in the broker via a new
oo-admin-ctl-cartridge command.  That cartridge can then be specified by name
at creation time.  Some additional cleanup to unit tests and application
also pave the way for future improvements to elaboration and cartridge tagging.

Adds a unique index on db.applications [domain_id, name] to do uniqueness at
the mongo level.

Fixes: Bug 1052829 - Made help and man page identical for oo-admin-ctl-cartridge