schema/defs-config.json: Require rootfs properties and enumerate type
schema/defs-config.json: Require rootfs properties and enumerate type

The property had been (erroneously) "layers"; this commit fixes that
by making the property "type" and using an enum to require the value
to be "layers".  It also adds "required" entries for both
rootfs.diff_ids and rootfs.type.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/image-spec,jonboulle/image-spec,opencontainers/image-spec,mattspencer-arm/image-spec,jonboulle/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec,vbatts/oci-image-spec