[GSS](7.0.z) :read-proxies-configuration and :read-proxies-info fail when at least one of the proxies is unreachable

[domain@localhost:9999 subsystem=modcluster] pwd
/host=localhost/server=cluster2/subsystem=modcluster

[domain@localhost:9999 subsystem=modcluster] :list-proxies
{
    "outcome" => "success",
    "result" => [
        "web02:8009",
        "web01:8009"
    ]
}

[domain@localhost:9999 subsystem=modcluster] :read-proxies-configuration
{
    "outcome" => "failed",
    "result" => undefined,
    "failure-description" => "JBAS014749: Operation handler failed: newValue is null",
    "rolled-back" => true
}

[domain@localhost:9999 subsystem=modcluster] :read-proxies-info
{
    "outcome" => "failed",
    "result" => undefined,
    "failure-description" => "JBAS014749: Operation handler failed: newValue is null",
    "rolled-back" => true
}