Traffic splitting WIP about knctl

[cppforlife]

related to knative/serving#1812

currenlty knative service objects imply that knative controls all aspects of route objects that it creates. hence when user changes route obj to route to different revisions, knative will quickly revert it back. there is ongoing discussion about how to change knative to allow user control of route but also keeping service object around. (currently one can do this by delete service obj and keeping configuration + route objects around).

[cppforlife]

proposed cli ux:

knctl route manage --route hello # change service to use `runLatest` key
knctl route unmanage --route hello # change service to use `manual` key

rollout command will also set route to be unmanaged

knctl rollout --route hello -r hello:latest=5% -r hello:previous=95%
knctl rollout --route hello -r hello:latest=10% -r hello:previous=90%
knctl rollout --route hello -r hello:latest=50% -r hello:previous=50%
knctl rollout --route hello -r hello:latest=100% -r hello:previous=0%
knctl rollout --route hello -s hello=100% -s hello=0%
knctl rollout --route hello --percent 5 # increase percentage of latest by X

[cppforlife]

implemented in v0.1.0 as knctl rollout command.