cri-containerd: hostpath mounted read only

[yvespp]

Description
Tried the cri-containerd runtime but the master node never becomes ready. In the logs of weave I can see that it can't write its configuration to disk because the file system is mounted read only.

Steps to reproduce the issue:
Used the master branch. Build the kube master like this: KUBE_RUNTIME=cri-containerd make master. Used ./boot.sh to boot it and then run kubeadm-init.sh
I'm on Mac OS 10.13.

Describe the results you received:
Weave tries to write it's configuration to a host path but fails:

linuxkit-0800279ca819:/# kubectl -n kube-system logs -f weave-net-h7gv9  weave
...
cp: can't create '/host/opt/cni/bin/weave-plugin-2.1.3': Read-only file system
/home/weave/weave: line 1576: can't create /host/etc/cni/net.d/10-weave.conf: Read-only file system
INFO: 2018/02/11 12:28:03.586451 Discovered local MAC 4a:7a:5a:07:dc:d1
INFO: 2018/02/11 12:28:04.427532 Weave version 2.2.0 is available; please update at https://github.com/weaveworks/weave/releases/download/v2.2.0/weave

The master node never becomes ready:

linuxkit-0800279ca819:/# kubectl describe nodes
Conditions:
  Type             Status  LastHeartbeatTime                 LastTransitionTime                Reason                       Message
  ----             ------  -----------------                 ------------------                ------                       -------
  Ready            False   Sun, 11 Feb 2018 13:12:00 +0000   Sun, 11 Feb 2018 12:23:48 +0000   KubeletNotReady              runtime network not ready: NetworkReady=false reason:NetworkPluginNotReady message:Network plugin returns error: cni config uninitialized

In the weave container I can see that the hostPath ist mounted ro:

linuxkit-0800279ca819:/# kubectl -n kube-system exec -it  weave-net-h7gv9  -c weave sh
/home/weave # mount | grep host
rootfs on /host/opt type tmpfs (ro,relatime)
rootfs on /host/home type tmpfs (ro,relatime)
rootfs on /host/etc type tmpfs (ro,relatime)
/dev/sda1 on /host/var/lib/dbus type ext4 (rw,relatime,data=ordered)
/dev/sda1 on /etc/hosts type ext4 (rw,relatime,data=ordered)

[ijc]

I spotted this when writing/running #58 too. I was testing a fix on Friday and hope to raise a PR today.