Cgroups memory used is not reported

Currently we don’t report cgroups mem used at all and sys_mem_limit (which is currently min(mem total, cgroup mem limit) doesn't make much sense, because from user's point of view it's super confusing. It's not clear what it can be compared to: host memory used or to cgroups memory used (which is not even reported now).

Instead I suggest we do the following:

• Add sys_mem_cgroup_used
• Add sys_mem_cgroup_limit
• Remove sys_mem_limit

Notes:
sys_mem_cgroup_limit will be missing completely if cgroups is not used or if memory cg controller is not used. If memory cg controller is there but the limit is not set, it will show 9223372036854771712 (0x7FFFFFFFFFFFF000))
sys_mem_cgroup_used will be missing completely if cgroups is not used or if memory cg controller is not used.

UPDATE:
Decided not to remove sys_mem_limit in 7.1.2 for backward compat reasons. Create MB-53124 for this.