parameterise hub ping frequency

[jihoonl]

https://github.com/robotics-in-concert/rocon_multimaster/blob/indigo/rocon_gateway/src/rocon_gateway/gateway_hub.py#L42

It should get applied to

• https://github.com/robotics-in-concert/rocon_multimaster/blob/indigo/rocon_gateway/src/rocon_gateway/gateway_hub.py#L59
• https://github.com/robotics-in-concert/rocon_multimaster/blob/indigo/rocon_hub_client/src/rocon_hub_client/hub_client.py#L92
• https://github.com/robotics-in-concert/rocon_multimaster/blob/indigo/rocon_hub_client/src/rocon_hub_client/hub_discovery.py#L175

[stonier]

We could, though I'm inclined not to unless there is a need (proliferation of parameters).

Also note there are two loops above, and probably should remain separate entities:

1. The hub discovery loop
2. The hub pinger (disconnection callback) loop

[stonier]

Reopen if there is a need for this.