Py3.4: Pass ConfigParser's extra kwargs through

In Python 3.4 ConfigParser has it's own string interpolation which recursively
calls get, but with extra kwargs, raw and fallback.

Accept these and pass them straight on to ConfigParser

airflow/configuration.py

@@ -302,7 +302,7 @@ def __init__(self, defaults, *args, **kwargs):
         self.defaults = defaults
         ConfigParser.__init__(self, *args, **kwargs)
 
-    def get(self, section, key):
+    def get(self, section, key, **kwargs):
         section = str(section).lower()
         key = str(key).lower()
         d = self.defaults
@@ -315,7 +315,7 @@ def get(self, section, key):
 
         # ...then the config file
         elif self.has_option(section, key):
-            return expand_env_var(ConfigParser.get(self, section, key))
+            return expand_env_var(ConfigParser.get(self, section, key, **kwargs))
 
         # ...then the defaults
         elif section in d and key in d[section]: