juju-core and goose update

[Ian Booth]

Time again to ensure that juju-core and goose are updated together.

Goose has had some updates to the service doubles to ensure compatibility with Canonistack.
juju-core has been updated to, by default, run the Openstack environ tests against the service doubles, which
is faster than using a real Openstack instance, and in the future, will allow us to more easily test using
different Openstack versions.