(PUP-2869) Puppet should support HTTP proxy authentication

Without this patch it is not possible to use the Puppet Module
Tool (PMT) with an authenticated proxy.
Many enterprises utilise the services of authenticated proxies for
Internet access. Current code is able to utilise a proxy, but does
not consider credentials for authenticated proxies.
This patch uses the credentials from either the environment settings
for HTTP_PROXY or http_proxy, or the corresponding values in the
puppet.conf file. The patch allows for http_proxy_user and
http_proxy_password to supplement the current http_proxy_host and
http_proxy_port variables within the puppet.conf.