APIcast should work with forward proxy

There is a common requirement from customers to be able to use APIcast in the environment where the external calls are made through a proxy (forward).
In such environment, APIcast can't work.

The workaround would be either to customize the Lua code, or to trick nginx on the DNS configuration level. But sometimes this can not be applied (customizations drop support, and such DNS change might not be allowed).

Some software systems use HTTP_PROXY / HTTPS_PROXY environment variables to specify the server that can be used as proxy. APIcast could use these variables, and point the requests to proxy instead of the real destination (while still keeping the original Host header).

UPDATE (2018-06-25): The feature should support forward proxy for the following:

• API backend (in case it's outside the user's network)
• Service Management API (3scale backend) – used for API authorization
• Account Management API (API manager / admin portal – https://\{DOMAIN}-admin.3scale.net) – used for pulling configuration

UPDATE (2018-07-03): Upstream