ipatests: test_otp: convert test_2fa_enable_single_prompt to run_ssh_cmd
    
    Paramiko is not compatible with FIPS.
    Migrate test_2fa_enable_single_prompt to the OpenSSH CLI SSH(1).
    Rationale: this is exactly what clients use.
    Also add a warning when test_2fa_disable_single_prompt is executed in FIPS mode.
    
    Fixes: https://pagure.io/freeipa/issue/8129
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Mohammad Rizwan <myusuf@redhat.com>
    Reviewed-By: Michal Polovka <mpolovka@redhat.com>