cgmanager: drop the cgm_supports_multiple_controllers bool

cgmanager: drop the cgm_supports_multiple_controllers bool

What we want is to make sure we dont' use controller 'all' if cgmanager
doesn't support, if all our cgroups aren't the same, or if we cannot
controll all our cgroups.  We were mixing some of these conditions.  Use
cgm_all_controllers_same for all.  (Might want to rename it, but we want
to stick with just one).

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>