SDK-17 Fixed external LoadBalanced config

[thibauult]

The external LoadBalanced was broken or has never worked as expected. This PR is more a patch than a real clean fix, knowing that the DFv2 will provide a real scalable solution "by design".

SymConfig.java

The method getAgentUrl() was using field this.agentHost to compute the full Agent server url. Instead, it has to use this.getAgentHost() because this method is overridden in SymLoadBalancedConfig.java.

SymLoadBalancedConfig.java

The way that the method public String getActualAgentHost() has been implemented was wrong :

String externalAgentHost = (agentServers != null && agentServers.size() > 0) ?
            agentServers.get(0) : getAgentHost();

It was apparently trying to get LB host from the Agent servers defined in the config-lb.json file. Moreover, having agentServers empty could have lead to a never-ending recursive loop with de subsequent call to getAgentHost()...

[symphonydarlys]

LGTM