Add option to mount ephemeral disks for image storage

[pieterlange]

Intended to be used on instances that come with local storage (eg the m3, c3,r3 andg2 instance types).

When just setting the enabled flag it formats (XFS) & mounts the local xvdb disk by default to the appropriate location (depending on container runtime). I chose XFS because of speed and the generous amounts of inodes by default.
Users can choose the filesystem and source disk by overriding the defaults:

experimental:
  ephemeralImageStorage:
    enabled: true
    disk: xvdc
    filesystem: ext4

⚠️ i haven't tested rkt as container runtime yet but i guess this will work? ⚔️

I would like to add the option to RAID together some local disks. The defaults should work all nodes with local storage, but doesn't use additional disks which is a waste if you move up to nodes of reasonable size (xlarge). Might add format/mount options later too.

Submitting incomplete work because @mumoshu seems to like rolling balls 😆 ⚽

[codecov-io]

Current coverage is 55.53% (diff: 100%)

Merging #95 into master will increase coverage by 0.21%

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[mumoshu]

Would you mind documenting it like not tested with rkt (yet) so we can be more explicit about what will/will not work 👍

[mumoshu]

I like this idea and yes, I like rolling balls! 🍙 💨
I've left a minor comment. Other than that, I'm ok to merge this.

[pieterlange]

Done 👍

[mumoshu]

LGTM 👍 Thanks for your continuous efforts 🙏

[tarvip]

Although this PR is already merged, but what about adding var-lib-docker.mount as dependency for docker.service, to make sure that it gets mounted before docker starts? Something like:

    - name: docker.service
      drop-ins:
{{if .Experimental.EphemeralImageStorage.Enabled}}
        - name: 10-docker-mount.conf
          content: |
            [Unit]
            After=var-lib-docker.mount
            Wants=var-lib-docker.mount
{{end}}

Using Wants allows using this drop-in even for instance types that do not have ephemeral storage available.

[pieterlange]

Sounds like a plan 👍 Will you make the PR? 😉