Provision new instances with version.Current

New instances were being provisioned with the agent-version from the
environ config instead of the currently running tools version. This
meant that there was a risk of new instances not running upgrade steps
if a juju upgrade occurred around the same time as the instance was
provisioned.
Merge pull request #383 from fwereade/provision-current-version

Provision new instances with version.Current

New instances were being provisioned with the agent-version from the
environ config instead of the currently running tools version. This
meant that there was a risk of new instances not running upgrade steps
if a juju upgrade occurred around the same time as the instance was
provisioned.