Update config-openstack documentation

Direct users to the Private Cloud howto for syncing simplestreams metadata in
order to successfully juju bootstrap.

It was no obvious that the "How to Private Cloud" was the next step to getting
juju working with Openstack.

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
Merge pull request #147 from battlemidget/patch-update-openstack-config

Update config-openstack documentation