Vagrant Openstack should works in degraded mode id only Keystone and Nova are availables

[ggiamarchi]

At the moment, the plugin always calls Neutron and Cinder to get information about networks and volumes. This makes possible to indicate in the Vagrantfile the name of a network rather than its id, for instance. If neutron or cinder api is not available, the plugin should works correctly if the Vagrantfile contains all the needed information for nova, i.e. only ids.

If the Vagrantfile configuration is valid regarding to the documentation but cannot be perfom because an api is unavailable, vagrant will fail with an explicit message that explain how to workaround the problem.