In deployments without an IPv4 gateway defined, for example when behind a proxy, Bifrost host variable file generation was failing. Fix by allowing ipv4_gateway to be set to an empty string, which is supported in Bifrost. Closes-Bug: #2133489 Change-Id: I988810541b4620ac196d216bf54896a774282fdd Signed-off-by: Pierre Riteau <pierre@stackhpc.com>