client: remove hard dependency on pam_krb5
    
    If ipa-client-install is executed with --no-sssd, check if pam_krb5 is
    available before proceeding with the install.
    
    https://fedorahosted.org/freeipa/ticket/5557
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>