Include kube-system migration steps in bootsrap

Notes:

I had to test things manually as there not an easy way to currently spin a minikube cluster for a single test. Filed https://github.com/weaveworks/launcher/issues/62 to fix that later.
- I added a "Executing migration from previous installation..." message as the deletion takes a long time (30s?)

The output of two successive bootstrap invocations on a cluster with WC agents in kube-system. Removed old agents from the kube-system namespace. You will have to reconfigure Deploy. only appears the first time.

$ ./build/bootstrap/bootstrap_linux_amd64 --scheme=http --hostname=192.168.99.100:30080 --token=<edited>

This will install Weave Cloud on the following cluster:
    Name: launcher-tests
    Server: https://192.168.99.100:8443

Please run 'kubectl config use-context' or pass '--kubeconfig' if you would like to change this.

Would you like to continue? [y/n]: y
Executing migration from previous installation...
Removed old agents from the kube-system namespace. You will have to reconfigure Deploy.
Storing the instance token in the weave-cloud secret...
Applying the agent...
Successfully installed. Please check the status at https://cloud.weave.works.

$ ./build/bootstrap/bootstrap_linux_amd64 --scheme=http --hostname=192.168.99.100:30080 --token=<edited>

This will install Weave Cloud on the following cluster:
    Name: launcher-tests
    Server: https://192.168.99.100:8443

Please run 'kubectl config use-context' or pass '--kubeconfig' if you would like to change this.

Would you like to continue? [y/n]: y
Executing migration from previous installation...
Storing the instance token in the weave-cloud secret...
A weave-cloud secret already exists. Would you like to continue and replace the secret? [y/n]: n
Cancelled.

weaveworks / launcher

Include kube-system migration steps in bootsrap #63