e2e: use DNS instead of HTTP to get my_public_ipv4

[gulducat]

ipify.org is frequently flakey, causing Terraform apply/destroy to errors in E2E tests, e.g.

│ Error: HTTP request error. Response code: 502
│ 
Terraform planned the following actions, but then encountered a problem:
│   with data.http.my_public_ipv4,
│   on network.tf line 23, in data "http" "my_public_ipv4":
│   23: data "http" "my_public_ipv4" {

This replaces the HTTP lookup with DNS, which is faster (not that speed matters a whole lot here), but more importantly, OpenDNS should (hopefully) be more reliable.

[shoenig]

LGTM!