Reduce informations from logging.bridge

Logging bridge maps 1-1 the log level from OSGi logging service to
slf4j. By design the OSGi framework set al INFO level all the modules
start and stop, which by pure chance, is also inline with our
guideline. Even though the compliance is satisfied there is a
usability issue because the console is flooded with many start stop
events given there are many bundles hence setting to WARN level the
log level for logging.bridge

Change-Id: I0dc484c49bd5966ad7dcdc01d55b9c28cfcf8217
Signed-off-by: Giovanni Meo <gmeo@cisco.com>