Make readonly metadata properties unable to edit

Image metadata properties os_hash_algo and os_hash_value are
readonly, and attempts to edit them fail with unclear message.

This patch makes these fields readonly in update metadata form.

Also, if the existing metadata property is null, Horizon should
consider it optional too, in addition to empty field fix:
https://review.opendev.org/c/openstack/horizon/+/812009

Change-Id: I892465ca4688fce9f7123682d02f11c92c7d2c5c
(cherry picked from commit 892080ec)