Make ipatests' create_external_ca a script
    
    The test helper create_external_ca is useful to create an external root
    CA and sign ipa.csr for external CA testing. I also moved the file into
    ipatests top package to make the import shorter and to avoid an import
    warning.
    
    Usage:
    
       ipa-server-install --external-ca ...
       python3 -m ipatests.create_external_ca
       ipa-server-install --external-cert-file=/tmp/rootca.pem \
           --external-cert-file=/tmp/ipaca.pem
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>