Add ability to change cgroup subsystem on running container.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)
Return output from get/set cgroup subsystem without tailing newline.

Remove unused comment.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)
Refactor cgroup settings.

Attach AddLXCConfig method to container struct.
Move IsInBytesSubsystem method to utils/utils.go.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)
Add test for dynamic cgroup subsystem settings.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)
Use gofmt to format code.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)
Add documentation for cgroup settings.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)
Refactor the cgroup code.

Remove api_params dependency.
Move top level cgroup.go to server.go.
Add UpdateConfig to execdriver interface to regenerate lxc config.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)
Move cgroup remote api doc from v1.7 to v1.9.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)
Allow to change cgroup with prefix 'lxc.cgroup'.

Docker-DCO-1.1-Signed-off-by: Jialin Tian <himurakenshin54@gmail.com> (github: kenshin54)