adds check on config_format kwarg in junos

This adds a check to validate the arugment for config_format kwarg in
get_config.  If the specified format is not a valid option, the shared
module will call fail_json

lib/ansible/module_utils/junos.py

@@ -250,9 +250,12 @@ class Netconf(object):
         return self.device.facts
 
     def get_config(self, config_format="text"):
-        ele = self.rpc('get_configuration', format=config_format)
+        if config_format not in ['text', 'set', 'xml']:
+            msg = 'invalid config format... must be one of xml, text, set'
+            self._fail(msg=msg)
 
-        if config_format == "text" or config_format == "set":
+        ele = self.rpc('get_configuration', format=config_format)
+        if config_format in ['text', 'set']:
            return str(ele.text).strip()
         elif config_format == "xml":
             return ele