Build: Improve integ test to match ant behavior

Many other improvements:
* Use spaces in ES path
* Use space in path for plugin file installation
* Use a different cwd than ES home
* Use jps to ensure process being stopped is actually elasticsearch
* Stop ES if pid file already exists
* Delete pid file when successfully killed

Also, refactored the cluster formation code to be a little more organized.

closes elastic#14464

Author: rjernst

buildSrc/src/main/groovy/org/elasticsearch/gradle/plugin/PluginBuildPlugin.groovy

@@ -49,7 +49,7 @@ class PluginBuildPlugin extends BuildPlugin {
             project.integTest.configure {
                 dependsOn project.bundlePlugin
                 cluster {
-                    plugin 'installPlugin', project.bundlePlugin.outputs.files
+                    plugin project.name, project.bundlePlugin.outputs.files
                 }
             }
         }

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

@@ -41,25 +41,18 @@ class ClusterConfiguration {
 
     Map<String, String> systemProperties = new HashMap<>()
 
-    @Input
-    void systemProperty(String property, String value) {
-        systemProperties.put(property, value)
-    }
+    LinkedHashMap<String, FileCollection> plugins = new LinkedHashMap<>()
 
     LinkedHashMap<String, Object[]> setupCommands = new LinkedHashMap<>()
 
     @Input
     void plugin(String name, FileCollection file) {
-        setupCommands.put(name, ['bin/plugin', 'install', new LazyFileUri(file: file)])
+        plugins.put(name, file)
     }
 
-    static class LazyFileUri {
-        FileCollection file
-        @Override
-        String toString() {
-            return file.singleFile.toURI().toURL().toString();
-        }
-
+    @Input
+    void systemProperty(String property, String value) {
+        systemProperties.put(property, value)
     }
 
     @Input