This fix tries to address the proposal raised in 27921 and add
`--cpus` flag for `docker run/create`.
Basically, `--cpus` will allow user to specify a number (possibly partial)
about how many CPUs the container will use. For example, on a 2-CPU system
`--cpus 1.5` means the container will take 75% (1.5/2) of the CPU share.
This fix adds a `NanoCPUs` field to `HostConfig` since swarmkit alreay
have a concept of NanoCPUs for tasks. The `--cpus` flag will translate
the number into reused `NanoCPUs` to be consistent.
This fix adds integration tests to cover the changes.
Related docs (`docker run` and Remote APIs) have been updated.
This fix fixes 27921.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
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