shutils/cgroups: fix run_into support

In the previous patch:
  cgroups: Mount cgroups controllers in devlib working dir
we changed the default mount point for devlib managed CGroups but forgot to
update the support for execution of a workload within a specified CGroup.

The run_into support is provide by a shutil script, which still has hardcoded
the old path (i.e. /sys/fs/cgroup/devlib_*). This patch fixes this by:
- resetting the default path to the Linux standard /sys/fs/cgroup
- use-sing the existing CGMOUNT env variable to specify which CGroups mount
  point to use

The cgroups::run_into is also updated to use the self.cgroup_root via
CGMOUNT when the shutils' script is called.
Moreover, an additional cgroups::run_into_cmd method is added which just
returns a properly formatted run_into shutils' call.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
diff --git a/devlib/bin/scripts/shutils.in b/devlib/bin/scripts/shutils.in
index 640d65d..fa4ad93 100755
--- a/devlib/bin/scripts/shutils.in
+++ b/devlib/bin/scripts/shutils.in
@@ -102,7 +102,7 @@
 cgroups_run_into() {
 
 	# Control groups mount point
-	CGMOUNT=${CGMOUNT:-/sys/fs/cgroup/devlib_*}
+	CGMOUNT=${CGMOUNT:-/sys/fs/cgroup}
 	# The control group we want to run into
 	CGP=${1}
 	shift 1