update doc for ca-show and ca-sign
update doc for ca-show and ca-sign

The usage of ca-show and ca-sign have been changed in [1].
So we should update the doc.

[1] https://review.openstack.org/#/c/303272/

Change-Id: I14d98da2560e73e6d2f5cb64b089cc904974c8c5

openstack/magnum,ArchiFleKs/magnum,ArchiFleKs/magnum,jay-lau/magnum,openstack/magnum