Add App Engine Deploy to Gradle build

Add a first pass add Nomulus GAE deployment to our gradle build scripts.  This
attempt could stand some improvement.  In particular:

- It currently only deploys to crash, per discussion we will make the
  environment a parameter.
- This uses a different import mechanism from the other plugins (via a
  "buildscript" section) and it looks like the lint plugin is very similar.
  I'll experiment with getting it to work that way.
- There is a lot of redundancy in the deployment scripts for each module, and
  given that we have the full power of a general programming language it
  should be possible for us to reuse the common parts.

But that said, this is a pretty good first step and I wanted to put it out
there.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=223366370

gradle/backend/build.gradle

@@ -0,0 +1,28 @@
+plugins {
+  id 'war'
+}
+
+// Set this directory before applying the appengine plugin so that the plugin
+// will recognize this as an app-engine standard app (and also obtains the
+// appengine-web.xml from the correct location)
+project.convention.plugins['war'].webAppDirName =
+    '../../java/google/registry/env/crash/backend'
+
+apply plugin: 'com.google.cloud.tools.appengine'
+
+// Get the web.xml file for the service.
+war {
+  webInf {
+    from '../../java/google/registry/env/common/backend/WEB-INF'
+  }
+}
+
+appengine {
+  deploy {
+    project = 'domain-registry-crash'
+  }
+}
+
+dependencies {
+  compile project(':core')
+}