subscription-manager proxy entries are not persistent in rhsm.conf

[snagoor]

Example:

Client <----> Proxy <----> Capsule.

In this situation, passing in the RHSM proxy options, --proxy=PROXY_URL , --proxyuser=PROXY_USER and --proxypassword=PROXY_PASSWORD are not persistent after the host is registered.

Proxy settings are mandatory in this case as any subscription-manager requests must go through a proxy, as direct communication with the capsule isn't allowed.

[snagoor]

PR opened for this #326