[cgroups2] Enable support for nested containers in the isolator.

We enable support for nested containers on systems with cgroups v2 with
this patch. This means that nested containers will now have their
cgroups created for them, and that the cgroups2 isolator functions will
be called for nested containers.

Review: https://reviews.apache.org/r/75177/

src/slave/containerizer/mesos/isolators/cgroups2/cgroups2.cpp

@@ -162,8 +162,7 @@ Try<Isolator*> Cgroups2IsolatorProcess::create(
 
 bool Cgroups2IsolatorProcess::supportsNesting()
 {
-  // TODO(dleamy): Update this once cgroups v2 supports nested containers.
-  return false;
+  return true;
 }