[DOP-39630] Hide secrets from settings repr

[dolfinus]

Change Summary

Discovered in MTSWebServices/horizon#227

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[github-actions]

Coverage Report •

File	Stmts	Miss	Branch	BrPart	Cover	Missing
syncmaster/db/migrations
env.py	62	62	14	14	0%	3–5, 7–13, 15–16, 23, 26–28, 31–32, 34, 37, 39, 45–47, 49–50, 52–59, 61–62, 64, 66, 69–73, 76, 88–89, 97–98, 101–102, 108–109, 112, 118, 124–125, 127, 130, 133, 136–137, 139
syncmaster/server
__init__.py	35	3	0	0	91%	67–69
syncmaster/server/providers/auth
dummy_provider.py	61	25	6	4	57%	54–56, 58–63, 65–68, 70–72, 79–80, 84, 89, 99–101, 111, 115
keycloak_provider.py	103	17	14	3	83%	66, 83–85, 119–121, 123->124, 124–125, 131->132, 132–133, 161->162, 162–163, 167–170
oauth2_gateway_provider.py	56	14	6	2	74%	47->48, 48–50, 54–57, 67->68, 68–69, 79–80, 99, 112, 118
syncmaster/server/scripts
manage_superusers.py	80	31	20	10	59%	53->54, 54–56, 73->74, 74–76, 92–93, 95–97, 99–101, 103–104, 106, 110–111, 113, 115, 119–120, 122–125, 127–128, 130–131
syncmaster/server/settings/auth
__init__.py	15	3	2	1	76%	42->43, 43–44, 49
syncmaster/settings
broker.py	18	5	2	1	70%	12->13, 13–14, 52, 55–56
database.py	26	11	4	4	50%	12–14, 16–19, 21, 74, 77–78
TOTAL	5403	653	838	177	87%