Allow configuring an alternative file as the `app.yaml`

[briandealwis]

The docs note that the user may choose to rename their app.yaml files:

You can specify a unique name for your app.yaml files, but then you must specify the file name with the deployment command as well. For example, if you name your app.yaml file service-name-app.yaml or app.flexible.yaml, […]

We could allow the user to "bless" a particular file as the app.yaml (called the service deployment descriptor in the docs) via a right-click option, and we store and retrieve this information somehow.

Or we could allow Deploy to App Engine to be an right-click option for .yaml files that seem to encode a service deployment descriptor files.

[chanseokoh]

We need to first ensure if every stakeholder in our tool chain can work with different names for app.yaml. This isn't the case. CT4E and appengine-plugins-core don't allow this.

AppYamlValidator:

    // appengine-plugins-core does not yet support other file names.
    if (!"app.yaml".equals(appYaml.getName())) {
      return ValidationStatus.error(Messages.getString("error.app.yaml.invalid.name", appYaml));

AppEngineProjectDeployer:

    deployables.add(stagingDirectory.append("app.yaml").toFile()); //$NON-NLS-1$