Return the proxy servers available.

        First env variables will be searched and updated with values from
        condarc config file.

def proxy_servers(self):
        """
        Return the proxy servers available.

        First env variables will be searched and updated with values from
        condarc config file.
        """
        proxy_servers = {}
        if self._load_rc_func is None:
            return proxy_servers
        else:
            HTTP_PROXY = os.environ.get('HTTP_PROXY')
            HTTPS_PROXY = os.environ.get('HTTPS_PROXY')

            if HTTP_PROXY:
                proxy_servers['http'] = HTTP_PROXY

            if HTTPS_PROXY:
                proxy_servers['https'] = HTTPS_PROXY

            proxy_servers_conf = self._load_rc_func().get('proxy_servers', {})
            proxy_servers.update(proxy_servers_conf)

            return proxy_servers