Cleanup from capabilities PR

[morgo]

This cleans up some small issues with capabilities, introduced in #4987

It modifies the bootstrap, which the CI system can not accurately test. Here is the proof that it runs without impact to the image:

$ make docker_bootstrap && make docker_test flavor=mysql57
..
2019/08/16 22:00:11 mysql57.unit_race                           FLAKY (1/2 failed)
2019/08/16 22:00:11 mysql57.update_stream                       PASS
2019/08/16 22:00:11 mysql57.update_stream_rbr                   PASS
2019/08/16 22:00:11 mysql57.vertical_split                      PASS
2019/08/16 22:00:11 mysql57.vertical_split_rbr                  PASS
2019/08/16 22:00:11 mysql57.vschema                             PASS
2019/08/16 22:00:11 mysql57.vtctld                              PASS
2019/08/16 22:00:11 mysql57.vtctld_web                          PASS
2019/08/16 22:00:11 mysql57.vtgate_buffer                       PASS
2019/08/16 22:00:11 mysql57.vtgate_utils                        PASS
2019/08/16 22:00:11 mysql57.vtgatev2                            PASS
2019/08/16 22:00:11 mysql57.vtgatev3                            PASS
2019/08/16 22:00:11 mysql57.vttest_sample                       PASS
2019/08/16 22:00:11 mysql57.worker                              PASS
2019/08/16 22:00:11 mysql57.xtrabackup                          PASS
2019/08/16 22:00:11 mysql57.xtrabackup_xbstream                 PASS
2019/08/16 22:00:11 ============================================================
2019/08/16 22:00:11 53 PASSED, 1 FLAKY, 0 FAILED, 0 SKIPPED
2019/08/16 22:00:11 Total time: 2h1m43.2s

Signed-off-by: Morgan Tocker tocker@gmail.com