Adds `disabled` field for instance-types.
Adds `disabled` field for instance-types.

The `disabled` field is intended to be used when phasing out
instance-types. In this case, a delete wouldn't work because the
instance-type needs to still be available for live instances using that
type, but we don't want to allow *new* instances created from that type.

In addition, we don't want to list hidden instance-types for regular
users, but we *do* want to list them for admin-users to ensure they have
a complete view of what's going on in the system.

Once all references to the phased-out instance-type have been dropped,
it would be safe to mark the instance-type as `deleted=True`.

Change-Id: I2af1c027f4d8114aee31353007dfdd3d0bb679ed

n0ano/gantt,n0ano/gantt