Add kube_dashboard_enabled label to user guide
Add kube_dashboard_enabled label to user guide

Following the introduction of the kubernetes dashboard deployment, add to the
user guide the label to be used to disable/enable this feature.

It's enabled by default.

Change-Id: I4c322beea84370dcd9544c8602011183be602f1e
Closes-Bug: #1718608

openstack/magnum,openstack/magnum,ArchiFleKs/magnum,ArchiFleKs/magnum