Update namespace finder and service validation checker

[dwlee]

To solve following issue
https://github.com/robotics-in-concert/rocon_tools/blob/indigo/rocon_python_comms/src/rocon_python_comms/services.py#L48

related PR
robotics-in-concert/rocon_qt_gui#182

[stonier]

is_valid_service is confusing...that assumes it is available AND not bad/corrupt.

is_available or service_is_available would be less confusing.

[jihoonl]

service_is_available

[jihoonl]

👍

[jihoonl]

I am changing the logic of find_namespace by following.

• renaming find_namespace to find_service_namespace - To be precise, it finds namespace for ros service. We may develop find_topic_namespace later.
• provide unique argument just as find_service and returns multiple namespaces - it is easier to handle code flow.
• service_type argument as optional - We may want to find namespace without type.

[jihoonl]

@stonier, @dwlee FYI