cfgupgrade: really ignore hostname when told so

cfgupgrade provides a --ignore-hostname option, however consequently it
spawns config.ConfigWriter, which refuses to handle a foreign
configuration file by default.

config.ConfigWriter provides a relative keyword argument,
accept_foreign, so we tie this to the value of options.ignore_hostname.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Reviewed-by: Michael Hanselmann <hansmi@google.com>