RendezvousIP should never be proxied

When a proxy is configured, users will almost never want to use it to proxy calls to the assisted-service API on the rendezvous host.

Currently, the agent installer uses the same proxy config in the Agent ISO as passed to assisted-service for the final cluster (assisted installs do the same in the discovery ISO, though with cause and effect reversed). Thus, the user must explicitly request the rendezvousIP to be excluded in the noProxy field of the install-config in order to avoid the proxy for communication between the agents and assisted-service as well as between the agent installer client running on the rendezvous host and assisted-service on the same host. In one customer case this caused difficult-to-debug problems because the proxy supported only GET and POST requests, not PATCH (used by the apply-host-config service).

When creating the cluster, assisted always adds the machine/service/cluster network CIDRs to the noProxy list in the generated install-config. So if it weren't for the communications with the rendezvousIP, users could expect to get away without having to specify any cluster IPs in the noProxy list.

Since the final cluster will have the proxy disabled for the machineNetwork IPs anyway, it seems safe to disable it for the rendezvousIP while the agent ISO is booted.