Collect all global ROS operations in service ros

The hydro-devel branch introduced the new `rostopic` global service that provides operations to create ROS streaming ConnPolicies. I would prefer to use the global service `ros` instead to collect all kind of ROS-related operations that not require to be loaded in a specific component. With this patch the rostopic plugin only installs operations in the global service "ros" instead of adding a new global service. The packages can (and must) still be imported independently:

```
import("rtt_ros") # installs global service "ros" and the ros.import() operation
ros.import("rtt_std_msgs") # imports rtt_roscomm package from rtt_std_msgs's <plugin_depend>
                           # and adds operations ros.topic, ros.topicBuffer and ros.topicUnbuffered

stream("foo.bar", ros.topic("bar"))
...
```

The main driver for this proposal is backwards compatibility with scripts generated for older versions of the toolchain. And I personally liked the `ros.topic(...)` syntax more than `rostopic.connection(...)`.

The ServiceRequester `rtt_rostopic::ROSTopic` would still work and uses the new names for the OperationCallers.

Just a suggestion. What do you think?