Rename EXTRA_ELEMENTS to SEED_DIB_EXTRA_ARGS

In boot-seed-vm we had a special variable that wasn't consistent with
the rest of devtest - it was only used there, not for other images we
build, and without namespaceing thats at best confusing. Also
cloud-init-nocloud is required for the seed, so there was no point
having it be optional.

Change-Id: Iecfba0cfa495c03c1d3e0495ec38274ee6407ab6
Merge "Rename EXTRA_ELEMENTS to SEED_DIB_EXTRA_ARGS"