Disable deployment with app setting SCM_DISABLE_DEPLOY_ON_PUSH

Fixes #1337
Though this would work in most cases, the consistent way to do this is via unified settings (see [this](https://github.com/projectkudu/kudu/blob/master/Kudu.Contracts/Settings/DeploymentSettingsExtension.cs)).  This would allow local kudu test without having to go thru env.