log-dir override on agent startup

[BrianJKoopman]

It'd be helpful to have a flag we can throw when starting an agent to override the log-dir configured for a host.

This is especially relevant for the Host Manager agent that is setup to control the Docker containers, since it is configured under the Docker "host", though runs directly on the host system. This means the context in which we need to configure the log directory is different for it compared to the rest of the agents in the configuration block.