Test: use trial license for security core rest tests

This commit adds the setting to use a trial license in the core rest
tests with security project. This is needed so that security is
actually enabled during these tests.

Original commit: elastic/x-pack-elasticsearch@b08bdaeb6f

qa/core-rest-tests-with-security/build.gradle

@@ -26,6 +26,7 @@ integTestCluster {
   setting 'xpack.watcher.enabled', 'false'
   setting 'xpack.monitoring.enabled', 'false'
   setting 'xpack.ml.enabled', 'false'
+  setting 'xpack.license.self_generated.type', 'trial'
   setupCommand 'setupDummyUser',
                'bin/x-pack/users', 'useradd', 'test_user', '-p', 'x-pack-test-password', '-r', 'superuser'
   waitCondition = { node, ant ->