Ignore case in yes/no prompts

We did not accept answers like "Yes", "YES", "No", etc. as valid
answers to yes/no prompts (used for example in dnsrecord-del
interactive mode). This could confuse users. This patch changes
the behavior to ignore the answer case.

https://fedorahosted.org/freeipa/ticket/2484

ipalib/cli.py

@@ -574,7 +574,7 @@ class textui(backend.Backend):
             prompt = u'%s Yes/No: ' % label
 
         while True:
-            data = self.prompt_helper(prompt, label)
+            data = self.prompt_helper(prompt, label).lower() #pylint: disable=E1103
 
             if data in (u'yes', u'y'):
                 return True