Add ingress controller configuration and backend to kubernetes clusters.
A new label 'ingress_controller' defines which backend should serve
ingress, with traefik added as the only option for now.
It is defined as a DaemonSet, with instances on all nodes defined with a
certain role. This role is set as an additional cluster label
'ingress_controller_role', with a default value of 'ingress'.
For now no node is automatically set with this role, with users or operators
having to do this manually after cluster creation.
Change-Id: I5175cf91f37e2988dc3d33042558d994810842f3
Closes-Bug: #1738808