Improve usage of optionals in FAST stage 2 VPN variables

[ludoo]

The current code requires to specify dev/prod/landing and their ASNs in order to set custom advertising even for a single environment. This PR adopts a more modern strategy which we use extensively in modules, and extends the use of optionals in the vpn_configs variable type, so the following code is not needed if only the landing advertisements need to be set.

vpn_configs = {
  # this needs to remain like defaults
  dev = {
    asn = 65501
  }
  landing = {
    # this needs to remain like defaults
    asn = 65500
    custom_advertise = {
      # this is what we really want to set
    }
  }
 # this needs to remain like defaults
  prod = {
    asn = 65502
  }
}