Performance improvement using in-operator on dicts

Just a small cleanup for the existing occurrences.

Using the in-operator for hash lookups is faster than using .keys()
http://stackoverflow.com/questions/29314269/why-do-key-in-dict-and-key-in-dict-keys-have-the-same-output

lib/ansible/modules/network/nxos/nxos_igmp_interface.py

@@ -1343,7 +1343,7 @@ def main():
 
     if state == 'absent':
         for each in CANNOT_ABSENT:
-            if each in proposed.keys():
+            if each in proposed:
                 module.fail_json(msg='only params: oif_prefix, oif_source, '
                                      'oif_routemap can be used when '
                                      'state=absent')