Add basic sanity checks

[fadenb]

Error: Duplicate declaration: Nginx::Resource::Vhost[app-name.dev] is already declared in file /tmp/vagrant-puppet-1/modules-1/component/manifests/symfony2.pp:20; cannot redeclare at /tmp/vagrant-puppet-1/modules-1/component/manifests/standalone_app.pp:12 on node app-name.dev

If you set standalone_app to true while symfony is still enabled the provisioning fails.
We should add some basic sanity checks for conflicting setups.