Fix project parameter in App Identity README.
Fix project parameter in App Identity README.

The gcloud command was getting the project argument when I was passing
it in as -Dgcloud_project. After changing to -Dgcloud.gcloud_project it
now deploys to the project I specify there. I hadn't noticed this until
I had modified my gcloud config and deployments started going to the
wrong project.

tswast/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,ErinJoan/mostly_empty_directory,jetty-project/java-docs-samples,tswast/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,ErinJoan/mostly_empty_directory,tswast/java-docs-samples,jetty-project/java-docs-samples,jetty-project/java-docs-samples,jetty-project/java-docs-samples,tswast/java-docs-samples,ErinJoan/mostly_empty_directory,jetty-project/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,ErinJoan/mostly_empty_directory