tools/cgdelete: add support for default systemd delegation slice/scope

tools/cgdelete: add support for default systemd delegation slice/scope

Enhance the cgdelete tool to support default systemd delegation
slice/scope, if set in the cgconfig.conf's systemd setting.  Parse the
configuration file and read the systemd::delegate setting, if any,
and set it as the default systemd slice/scope in systemd_default_cgroup.
Setting it appends the slice/scope name to the constructed default cgroup
mount path.

When the user passes the relative cgroup name, its appends to the
sub-tree (delegated slice/scope) by default and if the user wishes to
delete the cgroups in another subtree or cgroup root hierarchy, they
need to use the absolute path. For example:
		  cgroup_root
		 /	    \
		/	     \
	 systemd.slice*      cgrp1
	       |
	 systemd.scope
	       |
	    cgrp-d1

* default system delegation slice/scope - read from cgconfig.conf
$ sudo cgdelete -g cpu:cgrp-d1

will delete the cgroup cgrp-d1 under the
cgroup_root:systemd_default_cgroup:cgrp-d1

and for deleting the cgroup cgrp1, the user can use:
$ sudo cgdelete -g cpu:/cgrp1

or use the newly introduced -b switch to ignore the systemd slice/scope:
$ sudo cgdelete -b -g cpu:cgrp1

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>