Consolidate cgroup isolators into one single isolator.

There are two motivations for this:
1) It's very verbose to add a new isolator. For cgroup isolators (e.g., cpu, mem, net_cls, etc.), many of the logics are the same. We are currently duplicating a lot of the code.
2) Initially, we decided to use a separate isolator for each cgroup subsystem is because we want each subsystem to be mounted under a
different hierarchy. This gradually become not true with unified cgroup hierarchy introduced in kernel 3.16(The unified control group hierarchy in 3.16, cgroup-v2). Also, on some popular linux distributions, some subsystems are co-mounted within the same hierarchy (e.g., net_cls and net_prio, cpu and cpuacct). It becomes very hard to co-manage a hierarchy by two isolators.

We can still introduce subsystem specific code under the unified cgroup isolator by introduce a Subsystem abstraction. But we don't plan to support cgroup v2 in this ticket.