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
added env uuid to the state object
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.
envUUID on the state should be an environ tag
wip
keep the environ tag as a tag in our getter
wup
Merge pull request #401 from mattyw/add-uuid-to-state

added env uuid to the state object

Add the environment uuid to the state so that we can pass it on to AddEnvUser in a future pr
wip
configstore: use O_TRUNC when rewriting jenv file

Because thumper was determined not to write a temporary file then move it, we have to ensure that iff we rewrite the .jenv file, the contents of the existing file are truncated before writing to prevent future readers of the file seeing corrupt data at the end of the file.
Revert "wip"

This reverts commit 7ef024fb099177f16c75d22044bc94ee252ab8ae.
Revert "wup"

This reverts commit e56a4c0e4638bfbd1f5463a9708190c799c05d44.
Revert "wip"

This reverts commit 22b0fa3d7c305d869581e4e37e0a14670d348ddf.