[cob_bringup_sim] extend spawn_object: spawn models from database

In https://github.com/ros-simulation/gazebo_ros_pkgs/pull/261/files a launch file for spawning an object from the database is proposed: https://github.com/YoheiKakiuchi/gazebo_ros_pkgs/blob/add_spawn_launch_indigo/gazebo_ros/launch/spawn_model_in_database.launch

According to the usage of the gazebo_ros/spawn_model script, it is possible to use it in order to spawn models from the gazebo database. In the end, this scipts calls either gazebo's spawn_sdf_model or spawn_urdf_model via the gazebo_interface module which abstracts away the service calls.

However, in our script, we call the service directly...but only provide a client for the spawn_urdf_model service.

Things that need to be done in order to be able to spawn database models from our script:

[ ] replace service_clients from our script and make use of gazebo_interface calls instead. (?)
[ ] add a parameter parsing scheme to the yaml parsing scheme in our script in order to specify e.g. database as an model_type and then call the proper function from the gazebo_interface respectively...

This would reduce a lot of code duplications while still allowing us to use our object_locations.yaml parameter scheme!

@ipa-srd-rd Could you take this over? @ipa-nhg @ipa-mig @fmessmer FYI

ipa320 / cob_simulation

[cob_bringup_sim] extend spawn_object: spawn models from database #109