Store logging config in the environ config.

Store logging config in the environ config.

There are two main changes here.  Firstly is an environment variable for the
logging config.  This provides a source for the default logging config should
none be explicity set.  This is handled in two places: the Log command helper;
and the new config settings.

The second is to store the logging config in the environment config. This is
used in upcoming changes to configure the agents.

https://code.launchpad.net/~thumper/juju-core/logging-in-environment/+merge/185392

(do not edit description out of merge proposal)

[thumper, 2013-09-12 23:37:16]

Please take a look.

[wallyworld, 2013-09-12 23:47:39]

LGTM but I was confused when I read "environment", as in the Juju context, that
means something different to environment variables. eg we talk about environment
constraints as being stored in the environment, which means in state in mgo.
Could you consider changing the text to talk about environment variables?