chore: add TRAMPOLINE_IMAGE env var to config (#1096)

Author: rajatbhatta

.kokoro/continuous/integration-cloud-devel.cfg

@@ -33,3 +33,14 @@ env_vars: {
   key: "SECRET_MANAGER_KEYS"
   value: "java-client-testing"
 }
+
+# Configure the docker image for kokoro-trampoline.
+env_vars: {
+  key: "TRAMPOLINE_IMAGE"
+  value: "gcr.io/cloud-devrel-kokoro-resources/java8"
+}
+
+env_vars: {
+  key: "REPORT_COVERAGE"
+  value: "true"
+}