Use webproxy.$site.wmnet alias instead of carbon

carbon just happens to be our current forwarding proxy, but we have a
service alias per datacenter, so use that instead to be future-proof.

Change-Id: Idb0f720e09fdecee61581986f9a844017cb984b2
autoinstall: add http/proxy for private1-esams

private1-esams is a private subnet and hence needs a webproxy to
install.

Change-Id: Ic9cc07119cb64cee6c04f4cb3aa804b8a2c923f1