shipperctl: split "shipperctl admin clusters apply"

First of all, this does away with the "admin" namespace. Commands were
getting too long for our taste.

Second, `shipperctl admin clusters apply` is now split into two,
supposed to be called in order, like so:

```
shipperctl clusters setup management
shipperctl clusters join -f clusters.yaml
```

The first only installs Shipper in the management cluster, and the
second joins application clusters to the management one. This allows us
to roll out updates to shipper without necessarily having to specify
application clusters to be joined.

Makefile

@@ -94,10 +94,8 @@ install-shipper-mgmt: build/shipper-mgmt.image.$(IMAGE_TAG) build/shipper-mgmt.d
 # you should do when starting to work on shipper, as most of everything else
 # depends on having a management cluster talking to an application cluster.
 setup: $(SHIPPER_CLUSTERS_YAML) build/shipperctl.$(GOOS)-amd64
-	./build/shipperctl.$(GOOS)-amd64 admin clusters apply \
-		-f $(SHIPPER_CLUSTERS_YAML) \
-		--shipper-system-namespace $(SHIPPER_NAMESPACE) \
-		$(SETUP_FLAGS)
+	./build/shipperctl.$(GOOS)-amd64 clusters setup management -n $(SHIPPER_NAMESPACE) $(SETUP_FLAGS)
+	./build/shipperctl.$(GOOS)-amd64 clusters join -f $(SHIPPER_CLUSTERS_YAML) -n $(SHIPPER_NAMESPACE) $(SETUP_FLAGS)
 
 # Run all end-to-end tests. It does all the work necessary to get the current
 # version of shipper on your working directory running in kubernetes, so just

ci/clusters.yaml

@@ -1,5 +1,3 @@
-managementClusters:
-- name: kind-mgmt
 applicationClusters:
 - name: kind-app
   region: local