1) Changes to yang.bucklet to pass deps to yang_library
2) Addition of common model
3) Changes to l3vpn to refer to common model
4) Addition of common app to register all the common models
5) Add a flag in YangModel and make necessary changes to registration/unregistration to make sure dependent models are not registered/unregistered during referring application activation/deactivation.
6) Add remaining common file, after making required modification to l3vpn.