implement "-c" option to allocate a number of CPU shares to a container
if -c is not specified, do not set cpu.shares (instead of using the default value of 1024)
in the tests, use a non-default value for cpu.shares
propagate CpuShares in mergeConfig
Merge branch 'master' of github.com:dotcloud/docker into 471-cpu-limit
Merge pull request #551 from jpetazzo/471-cpu-limit

+ Runtime: implement "-c" option to allocate a number of CPU shares to a container