Speed up SouthboundIT through multi-threading test cases

Speed up SouthboundIT through multi-threading test cases

This speeds up SouthboundIT without losing coverage through the use of parallel
test cases.  On my VM,ExecutorSer this reduced test time from ~12 to ~4.5 minutes.

Aggregates the testing work for certain test cases into corresponding
Runnables.  For each set of test input data, a Runnable is spawned and
submitted to a fixed thread pool.  The thread pool size may be altered
through the NUM_THREADS constant.  The default is 4 since raising this
number causes issues with MD-SAL.

These test cases can be multi-threaded because they each utilize a distinct
bridge.  In other words, each thread is manipulating a different bridge in the
operating system.

Change-Id: Icd8ca69171e996fbf4000ef632c9446bb6c89ed6
Signed-off-by: Ryan Goulding <ryandgoulding@gmail.com>