Simplify environment variable parsing

[alexlovelltroy]

BSS currently requires three environment variables to be changed in order to configure the url encoded in the ipxe script. They can all be replaced with a single called BSS_IPXE_URL which includes the protocol and path. Overriding all of them is a bit cumbersome in Docker.

bss/cmd/boot-script-service/default_api.go

Line 71 in 9158f7e

var ipxeServer = getEnvVal("BSS_IPXE_SERVER", "api-gw-service-nmn.local")