HTTP(S) proxy can be specified when creating the template.
https://docs.openstack.org/magnum/latest/admin/magnum-proxy.html
However, it is not being utilized when talking to a public etcd discovery
service, which result in failed cluster creation. We need to be able to
use HTTP(S) proxy when services are running behind a firewall.
Change-Id: I13d86b0dc7c232a51149107f0412219388d8c2cd
story: 2004664