[r=wallyworld] Fix ensureGroup bug in Openstack provider
It appears not all Openstack providers return a duplicate error
if the security group already exists. Rework the business logic
to account for this. Also, if a group exists, do not overwrite its
rules with any new ones. This brings the juju-core behaviour in
line with what pyjuju does.
https://codereview.appspot.com/11655043/