keystoneclient endpoint-create should require service_id

It doesn't make sense to try to attempt to create an endpoint without referencing a service. Currently, this results in a server-side failure (a default value of None is assumed when it's not required).

    $ keystone endpoint-create
    Could not find service: None (HTTP 404)

It would make a bit *more* sense if service_id was the only *required* option, as region can be logically defaulted:

    $ keystone endpoint-create
    usage: keystone endpoint-create [--region <endpoint-region>] --service_id
                                    <service-id> [--publicurl <public-url>]
                                    [--adminurl <admin-url>]
                                    [--internalurl <internal-url>]
    keystone endpoint-create: error: argument --service_id is required

This successfully produces an endpoint (albeit without any URL's, but I think that's another issue being discussed on the mailing list (https://lists.launchpad.net/openstack/msg10322.html):

    $ keystone endpoint-create --service_id=34d34a251eea431cb2ca89c6dd360109
    +-------------+----------------------------------+
    | Property | Value |
    +-------------+----------------------------------+
    | adminurl | None |
    | id | fb6c1aab4ed04c94969fe6aef4670359 |
    | internalurl | None |
    | publicurl | None |
    | region | regionOne |
    | service_id | 34d34a251eea431cb2ca89c6dd360109 |
    +-------------+----------------------------------+