This fix tries to address the issue in raised #23367 where an out-of-band
volume driver deletion leaves some data in docker. This prevent the
reuse of deleted volume names (by out-of-band volume driver like flocker).
This fix adds a `--force` field in `docker volume rm` to forcefully purge
the data of the volume that has already been deleted.
Related documentations have been updated.
This fix is tested manually with flocker, as is specified in #23367.
An integration test has also been added for the scenario described.
This fix fixes#23367.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
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