test: Extend framework to handle custom goals easier

This commit adds a new option to setup_local_empty_lizardfs which can be
used like this:

  MASTER_CUSTOM_GOALS="8 X:  _ _ _ trash|14 bettergoal: _ _ _ trash trash"

to configure custom goals in a test's installation. This entry generates
a file with configuration of goals and adds its location to the config
file of all metadata servers.

Change-Id: I0595cc270d9b4738234b6fbbfdd65ae39ae088aa