create_node fails on Openstack driver when Nova enable_instance_password=false

For the Openstack driver. As noted in:

http://docs.openstack.org/essex/openstack-compute/admin/content/configuring-compute-API.html#d6e1833

Nova can be configured via the "enable_instance_password" parameter to not bother trying to inject the admin password for Hypervisors that do not support that (by default it does try to). In this case, the response from Nova does not include the adminPass parameter, so the create_node function fails while parsing the response.

(trival) patch follows.