If no swap is specified default to 1MB

After a244d1bc7b the Nova Ironic Driver will add the pxe_swap_mb parameter
using the value in the flavor (which defaults to 0 if not specified). In
the Ironic code we had something like get('pxe_swap_mb', 1) to prevent
the swap size for being 0, but that get() won't work anymore because the
pxe_swap_mb is set. So this patch is taking another approach to force
the the pxe_swap_mb parameter to be > 0.

Related-Bug: #1297937
Change-Id: Ie124f58681fc184950918cde1bc8c565c1387fc8
Merge "If no swap is specified default to 1MB"