gabrieljackson: Add `set instancegroup` command

JobBuild
10593
01-25 02:28
pull-kops-bazel-build 1353584808743145472 1353530049340379136
pull-kops-bazel-test 1353584808776699904 1353530049373933568
pull-kops-e2e-k8s-containerd 1353584808843808768 1353530049457819648
pull-kops-e2e-kubernetes-aws 1353584808814448640 1353530049420070912
pull-kops-verify-bazel 1353584808881557504 1353530049491374080
pull-kops-verify-boilerplate 1353584808969637888 1353530049613008896
pull-kops-verify-generated 1353584808910917632 1353530049541705728
pull-kops-verify-gofmt 1353584809003192320 1353530049642369024
pull-kops-verify-gomod 1353584808944472064 1353530049575260160
pull-kops-verify-govet 1353584809028358144 1353530049680117760
pull-kops-verify-staticcheck 1353584809573617664 1353530050175045632