Add --cpus flag to control cpu resources

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>

docs/reference/commandline/create.md

@@ -35,6 +35,7 @@ Options:
       --cpu-period int              Limit CPU CFS (Completely Fair Scheduler) period
       --cpu-quota int               Limit CPU CFS (Completely Fair Scheduler) quota
   -c, --cpu-shares int              CPU shares (relative weight)
+      --cpus NanoCPUs               Number of CPUs (default 0.000)
       --cpu-rt-period int           Limit the CPU real-time period in microseconds
       --cpu-rt-runtime int          Limit the CPU real-time runtime in microseconds
       --cpuset-cpus string          CPUs in which to allow execution (0-3, 0,1)

docs/reference/commandline/run.md

@@ -33,6 +33,7 @@ Options:
       --cpu-period int              Limit CPU CFS (Completely Fair Scheduler) period
       --cpu-quota int               Limit CPU CFS (Completely Fair Scheduler) quota
   -c, --cpu-shares int              CPU shares (relative weight)
+      --cpus NanoCPUs               Number of CPUs (default 0.000)
       --cpu-rt-period int           Limit the CPU real-time period in microseconds
       --cpu-rt-runtime int          Limit the CPU real-time runtime in microseconds
       --cpuset-cpus string          CPUs in which to allow execution (0-3, 0,1)