in blockstore-related VGs.

Right now, you have to decide globally and in advance, what disk types
are going to be included in blockstore pools. Then you set the sitevar
accordingly and then set the DB sysvol/nonsysvol/any node_type_features
to reflect the amount of storage available on just drives of that type.

This value is passed to clients via the otherwise unused PROTO field
of the blockstore line (when CMD=SLICE and CLASS=local), so this change
is backward compatible (OS images with older client code will ignore it
and just give you blockstores including all the devices).

So at Wisconsin, I set storage/local/disktype to "HDD-only" and tweak
the node_type_attributes '?+disk_any' and '?+disk_nonsysvol' to not
include the space for the 1 or 2 SSD drives in each machine. tmcd passes
the PROTO=HDD-only value and the client sees that and does not include
any SSD devices among the eligible devices from which to create the VG.

The hope is that ultimately, we could get rid of the sitevar and use the
PROTO field to select, per-blockstore, its type (only HDD, only SSD).
But that will require additional per node (type) assign features
differentiating the amount of each type available.