Set external nodes to default to 512m of heap

This commit sets external nodes for integration tests to default to
using 512m of heap. This can be overridden using tests.heap.size (a
system property that we already use elesewhere for setting the size of
the heap for the test runner) or using tests.jvm.argline (this last one
takes precedence).

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy

@@ -54,7 +54,9 @@ class ClusterConfiguration {
     boolean debug = false
 
     @Input
-    String jvmArgs = System.getProperty('tests.jvm.argline', '')
+    String jvmArgs = "-Xms" + System.getProperty('tests.heap.size', '512m') +
+            " " + "-Xmx" + System.getProperty('tests.heap.size', '512m') +
+            " " + System.getProperty('tests.jvm.argline', '')
 
     /**
      * The seed nodes port file. In the case the cluster has more than one node we use a seed node