Make revocation_reason required when revoking a certificate.

This will prevent errors if an empty reason is provided and it is
set by default one doesn't have to always set it on the command-line.

https://fedorahosted.org/freeipa/ticket/2597

ipalib/plugins/cert.py

@@ -526,7 +526,7 @@ class cert_revoke(VirtualCommand):
 
     # FIXME: The default is 0.  Is this really an Int param?
     takes_options = (
-        Int('revocation_reason?',
+        Int('revocation_reason',
             label=_('Reason'),
             doc=_('Reason for revoking the certificate (0-10)'),
             minvalue=0,