Do not contact AWS in offline mode (elastic/x-pack-elasticsearch#1001)

This commit restores the ability to build x-pack-elasticsearch without issues when running without
access to the internet. When the `--offline` flag is used, we will not try to contact vault and the
aws apis to retrieve the ml-cpp binaries but instead gradle will use a cached version even though
it may be expired.

relates elastic/x-pack-elasticsearch#726

Original commit: elastic/x-pack-elasticsearch@b0915d8fa9

plugin/build.gradle

@@ -130,8 +130,15 @@ gradle.taskGraph.whenReady { taskGraph ->
 
   if (taskGraph.hasTask(bundlePlugin)) {
     if (project.hasProperty("mlAwsAccessKey") == false && project.hasProperty("mlAwsSecretKey") == false) {
+      if (project.gradle.startParameter.isOffline()) {
+        // if the project is offline, then we shouldn't try to contact AWS and instead can just
+        // use a cached, but possibly expired artifact so that the build works...
+        project.ext.mlAwsAccessKey = ""
+        project.ext.mlAwsSecretKey = ""
+      } else {
         setAwsCreds()
       }
+    }
 
     repositories {
       maven {