Add CloudCredentialTag

[axw]

We add a new tag for identifying cloud credentials
within a Juju controller. A credential tag has three
components: the credential name, its owner (a valid
user), and the cloud to which the credential pertains.

(Review request: http://reviews.vapour.ws/r/5437/)

[axw]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju-names

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju-names/32

[axw]

$$wat$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju-names

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju-names/33

[axw]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju-names