Add support for glpi_agents - remote_addr inventory request

Add support for glpi_agents - remote_addr inventory request.

For NAT based / VPN Network, public remote address could be used to contact agent, and request inventory.

in agent.php Agent::getAgentURLs(), Agent::guessAddresses() only retrieve URL based on glpi_networkports linked with computer, and not glpi_agents public address stored in glpi_agents remote_addr.