Deploy command should not build APP_URL if there is no app

Discovered this while testing LC Solr deploys. The Solr stacks don't have an app as everything they need is in cookbooks and chef_repo.

Looks like simple_deploy is building an invalid URL rather than my expectation of it not building one at all.
APP_URL=s3://-us-west-1//.tar.gz

I vote for skipping APP_URL if app is nil. We'll need to test how simple_deploy/stackster handles a nil APP_URL.