This fix tries to address the issue raised in 31032 where it was
not possible to specify `--cpus` for `docker update`.
This fix adds `--cpus` support for `docker update`. In case both
`--cpus` and `--cpu-period/--cpu-quota` have been specified,
an error will be returned.
Related docs has been updated.
Integration tests have been added.
This fix fixes 31032.
This fix is related to 27921, 27958.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
