pki: pki-enroll-openssh-cert.sh: do not allow interactive passphrase prompt
pki: pki-enroll-openssh-cert.sh: do not allow interactive passphrase prompt

passphrase is done in older ssh-keygen implementation in any error
condition. for example, if ca key cannot be read a passphrase prompt
will be issued.

feeding empty passphrase will workaround this for now.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1258365
Change-Id: I6da2a357b428778fa897392b48d06849ca8727e7
Signed-off-by: Alon Bar-Lev <2e3ceb4d37adc6ae3ccc00d3646e0e0a036ace03@redhat.com>
OpenUniversity/ovirt-engine,OpenUniversity/ovirt-engine,OpenUniversity/ovirt-engine,OpenUniversity/ovirt-engine,OpenUniversity/ovirt-engine