Rename Grenade's DEST to STACK_ROOT

This is to avoid confusion with DevStack's DEST which is set to
${STACK_ROOT}/$BASE_RELEASE in Grenade.

Change-Id: Ibc9b94bd51e7d5dd82ec45f23557923d2f38db96
Merge branch 'node-name-redux' into dt-test
Merge branch 'grenade-dest' into dt-test