Add more RHEL customizations to spec file
    
    - Handle name / alt name for Fedora and RHEL. On Fedora, the packages
      are named "freeipa-*" with alternative names "ipa-*". On RHEL it is
      the other way around.
    - Don't build ipatests on RHEL.
    - Use latest versions of KRB5 on RHEL
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>