make failure to connect to cgmanager DEBUG instead of ERROR

You can have both cgmanager and cgfs compiled in, and lxc will fall back
at runtime to cgfs if it cannot connect to cgmanager, so print the failure
to connect as a DEBUG like the code used to do.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>