Ephemeral data port

Made it so that master and worker supports ephemeral ports (aka, system finds a free port).  Was going to add data port config, but turns out that `tachyon.worker.data.port` was already set for this.  

I want to clean up the patch a bit more, so shouldn't be merged yet.  Also, this patch won't be enough to make tests run concurrently (need a thread-safe/ isolated config).
It may make sense to add a flag in CommonConf to indicate if the system is in testing mode. By default, the value should be false.

Then, add a check that the port should not be zero in non-testing mode.