base: cleanup KillbillConfigSource constructors

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
test: disable log4jdbc-log4j2 correctly for slf4j-simple

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
config: set default config parameters for log4jdbc-log4j2

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>