Fix sambamba, all config must derive from 1 key only: "sambamba" (for all sub commands)

Not required to have sambamba stick to a certain pipeline

public/biopet-extentsions/src/main/scala/nl/lumc/sasc/biopet/extensions/sambamba/Sambamba.scala

@@ -24,7 +24,7 @@ abstract class Sambamba extends BiopetCommandLineFunction {
 
   override def subPath = "sambamba" :: super.subPath
 
-  executable = config("exe", default = "sambamba", freeVar = false)
+  executable = config("exe", default = "sambamba", submodule = "sambamba")
   override def versionCommand = executable
   override def versionRegex = """sambamba v(.*)""".r
   override def versionExitcode = List(0, 1)