Allow for service_pass var to be overridden by attribute if set.

If node[:openstack][:paas][:service_pass] is set in a wrapper cookbook,
this allows it to be laid down accordingly.  Useful for enterprise adoptions
where you want to talk to a non-local auth source which requires authz.
Merge pull request #2 from rackerlabs/extend_solum_conf_mgmt

Allow for service_pass var to be overridden by attribute if set.