Change use of "potential veg" in FATES to "primary land"

[ekluzek]

Change uses of potential veg in FATES to reference "primary land" instead.

For example, change...

hlm_use_potentialveg

change to:

hlm_use_all_primary_land

and the comments around them that say "potential veg" use "primary land". Similar changes would happen in CTSM for the PVT system test.

Originally posted by @ekluzek in #1318 (comment)

[glemieux]

@ekluzek would like this to be completed by the ctsm6 code freeze: https://github.com/ESCOMP/CTSM/milestone/13