Fix deprecated config warnings

Fix deprecated config warnings

Checks for deprecated config values were being
done inside config.Validate(). However, this method
is called at least 3 times when an environment is
created, leading to duplicated warnings and incorrect
information being shown.

The deprecation checks are moved to Environs.Config(name)
which is only called once when an environment is
created.

https://code.launchpad.net/~wallyworld/juju-core/refactor-deprecated-config-processing/+merge/198330

(do not edit description out of merge proposal)

[wallyworld, 2013-12-10 01:26:22]

Please take a look.

[fwereade, 2013-12-11 09:28:58]

Not sure about this. I'll stay up tonight and talk when you get on, I think --
I'm wondering whether it's better to have a global deprecation warning manager
that suppresses duplicate complaints? I think I'd like to also see these
warnings in the server logs (that don't go via ReadEnvironsBytes)...

[fwereade, 2013-12-11 22:18:59]

On 2013/12/11 09:28:58, fwereade wrote:
> Not sure about this. I'll stay up tonight and talk when you get on, I think --
> I'm wondering whether it's better to have a global deprecation warning manager
> that suppresses duplicate complaints? I think I'd like to also see these
> warnings in the server logs (that don't go via ReadEnvironsBytes)...

Discussed live; high-level objections withdrawn. Get another review for the
details though:). Or I'll do it tomorrow morning.

[jameinel, 2013-12-12 06:32:30]

Its pretty clear this is just moving the code, so that part LGTM.

Is there a test we could have that would have caught that we were being overly
verbose about being unhappy about a deprecated field?

[wallyworld, 2013-12-12 10:27:38]

On 2013/12/12 06:32:30, jameinel wrote:
> Its pretty clear this is just moving the code, so that part LGTM.
> 
> Is there a test we could have that would have caught that we were being overly
> verbose about being unhappy about a deprecated field?

There's a new test in this code which more or less did that -
TestDeprecationWarnings
I added an explicit gc.HasLen check to ensure there is only one log message.