config: Make it easy to create default config

Previously a default config object was created by invoking a private
function, modifying the global config, before reading the configuration
from the config file. This design make it impossible to create a default
config object for testing, or to reload the configuration while vdsm is
running.

This patch replace the private function with public function accepting
a config object. Now you can create a default config object by creating
a new config parser and setting the defaults:

    fake_config = ConfigParser.ConfigParser()
    config.set_defaults(fake_config)
    fake_config.set('irs', 'repository', '/my/fake/repository')

Change-Id: I80ec628d1da5c0466af28cb73564c6e63fd5ca65
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25504
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>