replace the nat ami with nat gateway
replace the nat ami with nat gateway

- doing this to get around the really weird
aws bug where we seemingly cannot deploy
t3 or t2 mediums to specific AZs
- related to our previous attempt to fix the bug
tagged below

[#164253722]

Signed-off-by: Sara Montecino <555501a7693db1f3bf7728b595ce1a33dce3f67d@pivotal.io>

pivotal-cf-experimental/bosh-bootloader,pivotal-cf-experimental/bosh-bootloader