Description of problem:
ipa cert-request command help should enumerate all available request type

# ipa help cert-request
Usage: ipa [global-options] cert-request CSR-FILE [options]

Submit a certificate signing request.
Options:
  -h, --help            show this help message and exit
  --request-type=STR    <request_type>
  --profile-id=STR      Certificate Profile to use
  --ca=STR              Name of issuing CA
  --principal=PRINCIPAL
                        Principal for this certificate (e.g.
                        HTTP/test.example.com)
  --add                 automatically add the principal if it doesn't exist
                        (service principals only)
  --all                 Retrieve and print all attributes from the server.
                        Affects command output.
  --raw                 Print entries as stored on the server. Only affects
                        output format.

Version-Release number of selected component (if applicable):
ipa-server-4.4.0-14.el7_3.x86_64

How reproducible:
100%

Steps to Reproduce:
1. ipa cert-request --help

Actual results:
Help shows <request_type> for request type in ipa cert-request

Expected results:
Help should show pkcs10/12/7 or all supported request type in ipa cert-request
help