Use pre-JDK9 style FilePermissions on JDK9

JDK9 removed pathname canonicalization when constructing FilePermission objects, which breaks some of the FilePermissions added by
Elasticsearch. This commit adds the system property jdk.io.permissionsUseCanonicalPath which makes JDK9 behave like JDK8 w.r.t. FilePermissions (see
https://github.com/elastic/elasticsearch/issues/21534).

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -495,6 +495,8 @@ class BuildPlugin implements Plugin<Project> {
             systemProperty 'tests.artifact', project.name
             systemProperty 'tests.task', path
             systemProperty 'tests.security.manager', 'true'
+            // Breaking change in JDK-9, revert to JDK-8 behavior for now, see https://github.com/elastic/elasticsearch/issues/21534
+            systemProperty 'jdk.io.permissionsUseCanonicalPath', 'true'
             systemProperty 'jna.nosys', 'true'
             // default test sysprop values
             systemProperty 'tests.ifNoTests', 'fail'

plugins/discovery-azure-classic/build.gradle

@@ -67,7 +67,7 @@ task createKey(type: LoggedExec) {
     project.delete(keystore.parentFile)
     keystore.parentFile.mkdirs()
   }
-  executable = 'keytool'
+  executable = new File(project.javaHome, 'bin/keytool')
   standardInput = new ByteArrayInputStream('FirstName LastName\nUnit\nOrganization\nCity\nState\nNL\nyes\n\n'.getBytes('UTF-8'))
   args '-genkey',
           '-alias', 'test-node',

plugins/discovery-gce/build.gradle

@@ -35,7 +35,7 @@ task createKey(type: LoggedExec) {
     project.delete(keystore.parentFile)
     keystore.parentFile.mkdirs()
   }
-  executable = 'keytool'
+  executable = new File(project.javaHome, 'bin/keytool')
   standardInput = new ByteArrayInputStream('FirstName LastName\nUnit\nOrganization\nCity\nState\nNL\nyes\n\n'.getBytes('UTF-8'))
   args '-genkey',
           '-alias', 'test-node',