* pipeline: add option to display locked DVC stages.
The '--locked' option xplicitly displays only locked DVC stages.
Fixes#1864
* pipeline: support short option '-l' for displaying locked stages
* pipeline: minor cosmetic tweak
* pipeline: reformat code with black
* pipeline: test printing of locked stages
* pipeline: use caplog to ensure command outputs everything as expected
* pipeline: cleanup test
* pipeline: more strict comparison of test output