Initial cut for supporting Managed Updates

Summary

The following mpdev commands are added/updated:

# Publish a version of the deployer to a GCS bucket. The bucket stores metadata that points back to the deployer image.
mpdev publish --deployer_image=DEPLOYER_IMAGE --gcs_repo=GCS_REPO_PATH

# Install command now needs --gcs_repo for apps that support managed updates.
mpdev install --deployer=DEPLOYER_IMAGE --parameters=APP_PARAMETERS --gcs_repo=GCS_REPO_PATH

# Look at the available versions and update the app to the latest available one.
# Can also be used with --version to force update to a specific version.
mpdev update --name=APP_NAME --namespace=NAMESPACE [--version=SPECIFIC_VERSION]

Known problems

• mpdev verify does not yet work.