Description of problem:

In rhc we want the ability to support multiple cartridge additions with one command.  While we could implement this in the client it is error prone and would involve ordering cart additions based on dependencies and rolling back if one of the many calls to the broker fails.  This code should live in the broker as part of the REST API.  

examples:

rhc app create myapp ruby-1.9 mysql mongodb rockmongo
rhc cartridge add mysql mongodb rockmongo -a myapp

Order of cartridges should not matter as long as the list of cartridges can be successfully installed together.  The broker should reorder based on dependency issues.

On error the broker should be specific on what failed. e.g. The mysql cartridge failed to install; You specified rockmongo but it requires mongodb; No standalone cartridge was specified; The ruby-1.9 and ruby-1.8 cartridges conflict and can not be installed in the same application; etc.