Adds http proxy support for registering RHEL overcloud nodes

Adds http proxy support for registering RHEL overcloud nodes

It is quite common in large entreprises that direct HTTP/HTTPS to the outside
world is denied from nodes/systems but reaching out through a proxy is allowed.

This change adds support for an HTTP proxy when RHEL overcloud nodes reach
out to either the RHSM portal or to a satellite server. This allows the
overcloud nodes to download updates even in locked-down environments.

The following variables are settable through templates:
  rhel_reg_http_proxy_host:
  rhel_reg_http_proxy_port:
  rhel_reg_http_proxy_username:
  rhel_reg_http_proxy_password:

Note the following restrictions:
  - If setting rhel_reg_http_proxy_host,
    then rhel_reg_http_proxy_port cannot be empty.
  - If setting rhel_reg_http_proxy_port,
    then rhel_reg_http_proxy_host cannot be empty.
  - If setting rhel_reg_http_proxy_username,
    then rhel_reg_http_proxy_password cannot be empty.
  - If setting rhel_reg_http_proxy_password,
    then rhel_reg_http_proxy_username cannot be empty.
  - If setting either rhel_reg_http_proxy_username or
    rhel_reg_http_proxy_password, then rhel_reg_http_proxy_host
    AND rhel_reg_http_proxy_port cannot be empty

Closes-Bug: #1668618
Change-Id: I003ad5449bd99c01376781ec0ce9074eca3e2704
(cherry picked from commit 3002edc90a631f3adb8ae0ee696062347f94ea52)