Running upgrade steps for units

[Tim Penhey]

Hi William and Menno,

I've been investigating this bug:
	https://bugs.launchpad.net/juju-core/+bug/1494070

The upgrade steps were never planned to be executed for unit agents,
only on machine agents, but there are two upgrade steps identified in
this bug where they were clearly expecting to run for units.

Obviously there is a need to be able to run steps for units, but I don't
think we should add it to unit agents, especially given the desire to
merge the agents in the near future.

What I do think we should do is provide a helper function inside the
upgrade package that will run a step for each unit passing in the unit tag.

So what we really need is the ability for the API as connected to by a
machine to ask for a list of units that should be running on that
machine. Do we have an API or state function for this already? I haven't
found on in my cursory look just now, but I thought I'd raise this and
do a sanity check at the same time.

There would be clear limitations for the upgrade steps for the units as
the API connection as as a machine agent.  However both the upgrade
steps identified so far are just updating local disk state based on
other disk state for a specified unit.

Firstly does this sound like a reasonable approach?

Tim