Description of problem:

The hammer gpg sub-command was depreciated in favour of the new content-credentials sub-command, but it only supports gpg_key.

Please add support for ssl_cert option.

Version-Release number of selected component (if applicable):
~]# rpm -q satellite
satellite-6.8.0-1.el7sat.noarch
~]# hammer --version
hammer (2.1.2)

• hammer_cli_foreman (2.1.2.1)
• hammer_cli_foreman_admin (0.0.9)
• hammer_cli_foreman_ansible (0.3.2)
• hammer_cli_foreman_azure_rm (0.2.0)
• hammer_cli_foreman_bootdisk (0.3.0)
• hammer_cli_foreman_discovery (1.0.2)
• hammer_cli_foreman_docker (unknown version)
• hammer_cli_foreman_openscap (0.1.11)
• hammer_cli_foreman_remote_execution (unknown version)
• hammer_cli_foreman_tasks (unknown version)
• hammer_cli_foreman_templates (0.2.0)
• hammer_cli_foreman_virt_who_configure (unknown version)
• hammer_cli_katello (0.22.2.2)
  [root@dell-r330-12 ~]#

How reproducible:

every time

Steps to Reproduce:
1. ~]# hammer content-credential create --content-type --help
Could not create GPG key:
Error: Option '--key' is required.

See: 'hammer --help'

Actual results:
~]# hammer content-credential create --content-type --ssl_cert
Could not create GPG key:
Error: Option '--key' is required.

Expected results:

~]# hammer content-credential create --content-type --ssl_cert
prompts for file path