check for domain environment variable uniqueness on app create

The fuse cartridge has some baked in logic where it uses the existence/non-existence of some domain env variables to determinte if a new fuse app should create itself as the master, or join an existing fabric.  That results in race conditions today if 2+ fuse apps are created in the same domain simultaneously (each check, do not find the env var defined, and then attempt to define it as part of their install script).  I'm proposing a new option to create domain env variables "uniquely" and fail the app creation if the race condition occurs (ie domain env var already exists).

You seem to be persisting the "unique" flag for the env var in mongo as well. Do you anticipate a case where the cur_var["unique"] is true but the new_var["unique"] is false, even though the keys are the same?