Sender/AsyncSender: Support Hostname directive in agent config file

[rfdparker]

When a Sender/AsyncSender object is initialised with use_config=True and config_path as the path to a zabbix_agentd/zabbix_agent2 config file, the Hostname directive in the config file should be respected as a default host for sending values with the host set as - (or otherwise unspecified).

This would be consistent with the behaviour of zabbix_sender.