Support persistent local disk

[bzub]

Right now kube-metal boots via iPXE, run from RAM, and servers are intended to be ephemeral in nature. The advantages are faster provisioning times and a container-like workflow. Data can be persisted to the local hard drive, but it requires manual intervention after bootup.

We should support installing the OS to the local disk as an option as well. This will be better for longer-term clusters and more traditional workflows.

[bzub]

This was already implemented in 65c8550

core@controller-01 ~ $ findmnt -t btrfs           
TARGET                                                                                                 SOURCE                FSTYPE OPTIONS
/DATA                                                                                                  /dev/sda              btrfs  rw,relatime,seclabel,compress=lzo,ssd,space_cache,subvolid=5,subvol=/
/etc/ssl/etcd                                                                                          /dev/sda[/ssl-etcd]   btrfs  rw,relatime,seclabel,compress=lzo,ssd,space_cache,subvolid=263,subvol=/ssl-etcd
/home/core                                                                                             /dev/sda[/core]       btrfs  rw,relatime,seclabel,compress=lzo,ssd,space_cache,subvolid=261,subvol=/core
/etc/kubernetes                                                                                        /dev/sda[/kubernetes] btrfs  rw,relatime,seclabel,compress=lzo,ssd,space_cache,subvolid=260,subvol=/kubernetes
/var/lib/docker                                                                                        /dev/sda[/docker]     btrfs  rw,relatime,seclabel,compress=lzo,ssd,space_cache,subvolid=257,subvol=/docker
/var/lib/kubelet                                                                                       /dev/sda[/kubelet]    btrfs  rw,relatime,seclabel,compress=lzo,ssd,space_cache,subvolid=262,subvol=/kubelet
/var/etcd                                                                                              /dev/sda[/etcd]       btrfs  rw,relatime,seclabel,compress=lzo,ssd,space_cache,subvolid=259,subvol=/etcd
/var/lib/rkt                                                                                           /dev/sda[/rkt]        btrfs  rw,relatime,seclabel,compress=lzo,ssd,space_cache,subvolid=258,subvol=/rkt
`-/var/lib/rkt/pods/run/48a2cf99-04e5-4296-8708-44f667e69db3/stage1/rootfs/opt/stage2/etcd/rootfs/etc/ssl/certs
  /dev/sda[/ssl-etcd]   btrfs  ro,relatime,seclabel,compress=lzo,ssd,space_cache,subvolid=263,subvol=/ssl-etcd