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: I702336baaf0258f2ee40069889f7c3a0764a664e
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>