SBT plugin does not support paths to sbt-launcher.jar to be configured per node

The configuration of "sbt launcher jars" paths in the general jenkins configuration page (i.e. globally) should be overridable in "Tool location" for each node (like JDK, Git & Hg paths).

If running in a mixed environment with both linux and windows slaves, the path to sbt (like the other tools) will typically be very different on the two platforms (e.g. c:\sbt\sbt-launcher.jar vs. /usr/local/bin/sbt)