Allow absolute paths for configuration elements

The previous behaviour would only allow a function from within a module
to load a configuration element for it's own path.  E.g. Module:
hivemind_contrib.nova could load the config options
hivemind_contrib.nova.client using the decorator
@configurable('client').  But in some cases we need shared or abstract
configuration module paths.  This change supports this by allowing
explicit configurable paths
e.g. @configurable('nectar.openstack.client').

Change-Id: I39ce351201401d01da39aa9c3658045d16be530b
Merge "Allow absolute paths for configuration elements"