osd: add new CLS call to retrieve global configuration

osd: add new CLS call to retrieve global configuration

Currently there is no easy way to gain access to global configuration
from CLS (objclass) code. This adds a new call to the CLS interface
that returns a "const ConfigProxy&" from which configuration can be
accessed.

NOTE: Working code to do provide this functionality in crimson is not
provided.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>