Description of problem:
When no value is provided to --revocation-reason argument in ipa cert_revoke,
following unhelpful error message displayed.

[root@ipa63server ipa-cert]# ipa cert-revoke 5 --revocation-reason=
ipa: ERROR: an internal error has occurred
[root@ipa63server ipa-cert]

Version-Release number of selected component (if applicable):

[root@ipa63server ipa-cert]# rpm -q ipa-server
ipa-server-2.2.0-5.el6.x86_64
[root@ipa63server ipa-cert]#

How reproducible:
Always

Steps to Reproduce:
1.Install IPA server
2.Try to revoke certificate with 'ipa cert-revoke' and provide no value to
--revocation-reason optional argument.
[root@ipa63server ipa-cert]# ipa cert-revoke 5 --revocation-reason=
ipa: ERROR: an internal error has occurred
[root@ipa63server ipa-cert


Actual results:
Got following error message
"ipa: ERROR: an internal error has occurred"

Expected results:
Some helpful error message should be displayed