Put environment check in its separate task

The check for environment cannot be done when the task is defined,
otherwise running any task will fail when the check does not pass.
Instead, check the environment when the task executes.

appengine_war.gradle

@@ -71,3 +71,4 @@ dependencies {
 
 rootProject.deploy.dependsOn appengineDeploy
 rootProject.stage.dependsOn appengineStage
+appengineDeploy.dependsOn rootProject.verifyDeployment