provider/{ec2,local}: use environschema

[rogpeppe]

We add Schema methods to the providers to make the schema available
directly. In a future PR we will add the Schema method to the Provider
interface, but for the moment it's optional to avoid the need to change
all the providers at once.

(Review request: http://reviews.vapour.ws/r/1969/)

[rogpeppe]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/3719

[rogpeppe]

$$merge$$

[jujubot]

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