provider/manual: drop use-sshstorage

[axw]

We were using use-sshstorage in one place:
to decide whether ControllerInstances should
perform a host validity check or not. Instead
of doing that, we can just check if we're
running inside "jujud". With that in place,
drop use-sshstorage from config.

(Review request: http://reviews.vapour.ws/r/4259/)

[wallyworld]

$$merge$$

[jujubot]

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