yang:uuid type for interface-uuid should be string type

In the file:

sfc-model/src/main/yang/service-function-forwarder-logical.yang

The type of the interface-uuid should be string instead of yang:uuid in order to be consistent to Genius uuid type. On genius datastore and IETF Interfaces data store, both on which the configured interface will also be defined, the type is string.

Additionally the name of this corresponding field is interface-name, so it is also proposed to modify interface-uuid to interface-name.