cg: Add ability to dump custom cgroup properties

We have some common predefined properties such as "cpuset.cpus" and etc gathered
in @cgp_predefined set, but there might be situation when only predefined ones
are not enough, so add ability to specify properties via --cgroup-props
and/or --cgroup-props-file options.

For example one may pass file with content

"cpu":
 - "strategy": "merge"
 - "properties": ["cpu.shares", "cpu.cfs_period_us"]

to dump custom properties for cpu controller.

The description is implemented in almost valid yaml, probably we will
need to support the various forms, but oneline is enough for now.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>