This adds the commands needed to list and show nodegroups in a
cluster. Specifically the commands added are:
* openstack coe nodegroup list <cluster>
* openstack coe nodegroup show <cluster> <nodegroup>
Depends-On: I5607c27eb0e84677acda29af006335374b60dd27
Change-Id: I910c7c7caff34aba2ce08ca5c9362403d11f324c