External CA installer options usability fixes
The --external_cert_file and --external_ca_file options of ipa-server-install
and ipa-ca-install have been replaced by --external-cert-file option which
accepts multiple files. The files are accepted in PEM and DER certificate and
PKCS#7 certificate chain formats.
Reviewed-By: Petr Viktorin <firstname.lastname@example.org>