provider/openstack: don't fail if product-streams isn't defined

Fixes https://bugs.launchpad.net/juju-core/+bug/1371605
Merge pull request #809 from axw/lp1371605-provider-openstack-keystoneurl

provider/openstack: don't fail if product-streams isn't defined

Fixes https://bugs.launchpad.net/juju-core/+bug/1371605