CORE_PLUGINS doesn't allow disabling specific plugins

[Janiczek]

Describe the bug
If you set CORE_PLUGINS to a subset of the core plugins, eg. ("ACLs", "Backup", "Help"), the rest of the core plugins will error out periodically:

The only currently known way to remove those warnings is to enable all of the plugins.
Thus, there is no way to disable certain plugins without getting the periodic error messages.

To Reproduce
Set CORE_PLUGINS to a subset of the available core plugins.

Expected behavior
Only some plugins are enabled; no errors are shown.

Screenshots
See above.

Environment (please complete the following information):

• Errbot version: 6.1.9
• OS version: Alpine
• Python version: 3.9
• Using a virtual environment: yes
• Using Docker: yes

Additional context
-

[Janiczek]

Will likely be fixed by #1601

[andrewthetechie]

@sijis do you think a new fix release could get cut to include the fix for this bug? This is happening in 6.2.0 but doesn't happen if I install from git master