add property endpoint

In all private clouds, the user needs to supply the cloud endpoint url to connect to. Currently, Whirr has no means to supply this, outside diverting to jclouds-specific property "jclouds.endpoint" or "providername.endpoint"

The impact is that the CLI and other tools neither document, nor make easy supplying this. For example, a user should intuitively be able to supply --endpoint http://keystonehost/v2.0/ for openstack, but right now, it is essentially a secret!