Rename ispecs-disk-templates to ipolicy-disk-templates

The ispecs (min/std/max) are different from the other ipolicy
variables (disk templates for now, more in the future). As such, let's
rename the ispecs-disk-templates to ipolicy-disk-templates for
consistency with this logical difference.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>