freezer-api README.rst needs to be updated
freezer-api README.rst needs to be updated

Info of API registration in README.rst is not updated with the
latest openstack version. Change user-create/service-create/endpoint-create
from keystone command to openstack command.

Closes-bug: #1592242
Change-Id: Iff1924632835f92278e0cc7a666b8056655bd8e7

openstack/freezer-api,szaher/freezer-api,szaher/freezer-api,openstack/freezer-api,szaher/freezer-api,szaher/freezer-api,openstack/freezer-api,openstack/freezer-api