In initramfs, do not prompt if keylocation is "file://"

If the encryption key is stored in a file, the initramfs should not
prompt for the password. For example, this could be the case if the boot
partition is stored on removable media that is only present at boot time

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Garrett Fields <ghfields@gmail.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Sam Lunt <samuel.j.lunt@gmail.com>
Closes #9764