Make sure config.yang is MD-SAL enabled

Make sure config.yang is MD-SAL enabled

In the controller-controlling-controllers scenario we need to be able to
generate MD-SAL bindings against config subsystem models. In order to do
that, we need the base definitions provided by config.yang.

This patch subjects all config components to MD-SAL binding generation
and introduces a new parent, which adds another execution to generate
JMX bindings. Infrastructure components continue to use config-subsystem
parent while the various config plugins (netty et al.) switch to the new
config-plugin-parent.

Change-Id: Id243d658a95ccfee0dbfe70044867f8de09cc20e
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>