config: support conf.d notation for configuration

Read the config file and all config files within the .d directory sorted
by name.

It will make the configuration of the package simpler during
installation of 3rd party packages.

Change-Id: I09219762a7f548f41c58547e463153ea25f13034
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>