Implemented tf prefix for multiple robot / multiple camera systems

[wallner90]

Enables systems of multiple robots / multiple cameras to be simulated using one ROS-Core. tf of AR-tag has defined prefix per robot.

[130s]

Test failed, but looks irrelevant to this PR. I/someone need to take a look:

# BEGIN SUBSECTION: run tests
10:37:03 Invoking '. /opt/ros/lunar/setup.sh && . /tmp/catkin_workspace/devel_isolated/setup.sh && PYTHONIOENCODING=utf_8 PYTHONUNBUFFERED=1 catkin_make_isolated --force-cmake --cmake-args -DCATKIN_ENABLE_TESTING=1 -DCATKIN_SKIP_TESTING=0 -DCATKIN_TEST_RESULTS_DIR=/tmp/catkin_workspace/test_results --catkin-make-args -j1 run_tests' in '/tmp/catkin_workspace'
:
10:37:08 FAILURE:
10:37:08 [/tmp/catkin_workspace/src/ar_track_alvar/ar_track_alvar/test/marker_param_config-full.test]:
10:37:08 	Missing package dependencies: ar_track_alvar/package.xml: rostest, rosbag
10:37:08 wrote test file to [/tmp/catkin_workspace/test_results/ar_track_alvar/roslaunch-check__tmp_catkin_workspace_src_ar_track_alvar_ar_track_alvar_test_marker_param_config-full.test.xml]
:

Meanwhile, @wallner90 do you think you can add a simple testcase with a short .bag file? 10 seconds should be enough as long as image and all necessary topics are subscribable. You can refer to this PR about what's needed.

[130s]

Test failed, but looks irrelevant to this PR. I/someone need to take a look:

Similar error occurred in #39. So I asked on answers.

[AlexisTM]

This should not be merged: tf_prefix support is removed from TF2

http://wiki.ros.org/tf2/Migration#Removal_of_support_for_tf_prefix