Buttervolume

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- Now use the docker *managed plugin* system
- Stop the scheduler before shutdown to avoid a 5s timeout
- Improved logging
- Improved the migration doc from version 1 or 2

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- BREAKING CHANGE: Please read the migration path from version 1 to version 2:
BTRFS volumes and snapshots are now stored by default in different directories under ``/var/lib/buttervolume``
- Configuration possible through environment variables or a ``config.ini`` file
- implemented ``VolumeDriver.Capabilities`` and just return ``'local'``
- other minor fixes and improvements

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- Add clone command
- replace sync by `btrfs filesystem sync`

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- fixed packaging (missing README)

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- fixed the cli for the restore command

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- fixed the purge algorithm