server install: do not prompt for cert file PIN repeatedly
Prompt for PIN only once in interactive mode.
This fixes ipa-server-install, ipa-server-certinstall and
ipa-replica-prepare prompting over and over when the PIN is empty.
https://fedorahosted.org/freeipa/ticket/6032
Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>