Quick win - juju check

[Tim Penhey]

Hi folks,

We talked quite a bit in Vancouver about quick wins. Things we could get 
into Juju that are simple to write that add quick value.

There was also talk about "juju check".

The idea of this command is to surface things that should really be 
looked at.

I was thinking that we could get something in place, and iterate, rather 
than going for a gold plated command to start with.

Using the migration export command, we could easily do cross-entity checks.

The things that leap to mind are:
  * machines that only host units that don't have any units
  * applications (nee services) that have no units
  * machines that have been pending for more than X minutes
  * perhaps surfacing anything that is in an error state?

Thoughts?

Tim