GAE quickstart tutorial uses different appengine plugin 

In the GAE quick-start tutorial it uses:
```xml
<groupId>com.google.cloud.tools</groupId>
<artifactId>appengine-maven-plugin</artifactId>
<version>${appengine-maven-plugin-version}</version>
```

which is different from the one used by sample. We should switch either one of them to keep them consistent.