wip
environs/config: introduce config.UUID()

Introduce `config.UUID()` method to access the uuid of the environment.

A uuid is the canonical indentifier for an environment, and superceeds the environment's name.

For backwards compatability, `config.UUID()` is currently permitted to return "", but once all environments are converted to have an environment uuid in place, this method will change to return a valid uuid for a valid config.
Merge branch 'master' of https://github.com/juju/juju into 150-environs-config-grow-a-uuid-method
Permit uuid to be set if unset, otherwise refuse to set it.
Merge branch 'master' of https://github.com/juju/juju into 150-environs-config-grow-a-uuid-method
Merge branch 'master' of https://github.com/juju/juju into 150-environs-config-grow-a-uuid-method
Merge branch 'master' of https://github.com/juju/juju into 150-environs-config-grow-a-uuid-method
responding to review feedback
Merge pull request #378 from davecheney/150-environs-config-grow-a-uuid-method

environs/config: introduce config.UUID()

Introduce `config.UUID()` method to access the uuid of the environment.
    
A uuid is the canonical identifier for an environment, and superceeds the environment's name.
    
For backwards comparability, `config.UUID()` is currently permitted to return "", but once all environments are converted to have an environment uuid in place, this method will change to return a valid uuid for a valid config.