Add $java_home variable

On several places I needed to set the JAVA_HOME environment variable
for some software. To have this controlled in a central place I added
it here.

manifests/init.pp

@@ -52,6 +52,7 @@ class java(
     $default_package_name     = $java::params::java[$distribution]['package']
     $default_alternative      = $java::params::java[$distribution]['alternative']
     $default_alternative_path = $java::params::java[$distribution]['alternative_path']
+    $java_home                = $java::params::java[$distribution]['java_home']
   } else {
     fail("Java distribution ${distribution} is not supported.")
   }