tests: Make it easier to define labels for chunkservers

This commit adds a new parameter to the setup_local_empty_lizardfs
function which makes it possible to define labels for all chunkservers
like this:

  CHUNKSERVER_LABELS="0,1:de|2,3:us|4:cn"

This would make many tests of custom goals more concise.

Change-Id: I3ae4611f5a562f4827d157e24c6733dd22fd352e