Improved tf_prefix based on namespace

[kjvanteeffelen]

Solves #1048.

When using robot namespace as tf_prefix, it now removes any starting '/' and only adds an ending '/' if not present already.
Starting with '/' is not common in TF2 and showed compatibility issues with for instance robot_localization.

[saikishor]

The changes look fine to me!

Thank you!

[christophfroehlich]

Please have a look at the failing tests

[rafal-gorecki]

What is the problem with adding this fix? This issue has been going on almost for a year, and is it just a matter of a few lines of code.

[christophfroehlich]

Acc to my last comment, tests were failing. As this PR is locked for maintainers, I can't push something to it or merge the current master into it.
Doing that locally

    [  FAILED  ] TestDiffDriveController.configure_succeeds_tf_blank_prefix_true_set_namespace
    
     1 FAILED TEST
    -- run_test.py: return code 1
    -- run_test.py: inject classname prefix into gtest result file '/workspaces/ros2_rolling_ws/build/diff_drive_controller/test_results/diff_drive_controller/test_diff_drive_controller.gtest.xml'
    -- run_test.py: verify result file '/workspaces/ros2_rolling_ws/build/diff_drive_controller/test_results/diff_drive_controller/test_diff_drive_controller.gtest.xml'
  >>>
build/diff_drive_controller/test_results/diff_drive_controller/test_diff_drive_controller.gtest.xml: 17 tests, 0 errors, 1 failure, 0 skipped
- diff_drive_controller.TestDiffDriveController configure_succeeds_tf_blank_prefix_true_set_namespace (/workspaces/ros2_rolling_ws/src/ros2_controllers/diff_drive_controller/test/test_diff_drive_controller.cpp:392)
  <<< failure message
    /workspaces/ros2_rolling_ws/src/ros2_controllers/diff_drive_controller/test/test_diff_drive_controller.cpp:417
    Expected equality of these values:
      test_odom_frame_id
        Which is: "test_namespace/odom"
      test_namespace + "/" + odom_id
        Which is: "/test_namespace/odom"
  >>>

Maybe someone wants to fix this on top of this PR and open a new one.

[christophfroehlich]

Fixed with #1420