Added 'plaform' property to base provider class.

coriolis/providers/base.py

@@ -9,6 +9,10 @@ from coriolis import schemas
 class BaseProvider(object):
     __metaclass__ = abc.ABCMeta
 
+    @property
+    def platform(self):
+        raise NotImplementedError("Missing provider platform attribute.")
+
     @property
     def connection_info_schema(self):
         raise NotImplementedError("Missing connection info schema.")