cmd/jujud: agent should perform one cycle

cmd/jujud: agent should perform one cycle

Fixes #1068445.

This proposal alters the logic on the unit and machine agents to 
perform at least one cycle before checking the tomb value.

This change is important as our tests start an agent with a runStop()
method, who's intention is to run the agent for a short period of time
to exercise one aspect of its behavior.

The difficulty is runStop() can put the tomb into a Dying state before 
the agent has started, causing it to exit immediately without taking 
any action.

https://code.launchpad.net/~dave-cheney/juju-core/026-uniter-failure/+merge/131369

(do not edit description out of merge proposal)

[dave_cheney.net, 2012-10-25 10:31:13]

Please take a look.

[fwereade, 2012-10-25 14:55:38]

LGTM

[niemeyer, 2012-10-29 10:21:02]

The description says what this is doing, but it doesn't say why.

It's quite probably right, but what's the background for the change?

(I've read the bug too, but it only shows a log)

[dave_cheney.net, 2012-10-29 16:06:19]

Please take a look.

[dave_cheney.net, 2012-10-29 16:06:33]

PTAL, i have updated the description to better describe the problem and the
proposed solution.

[niemeyer, 2012-10-30 08:22:11]

Thanks. We discussed a slightly different approach to reach the same result (and
fix runStop, which is quite broken ATM).