[issue2551222] config.ini doesn't list all available databases

New submission from Tom Ekberg:

I was updating my config.ini moving from 2.1 to 2.2. I noticed that the line 

# Available backends: anydbm, sqlite

near line 572 was missing databases.

I know that mysql and postgresql are available, but are not shown in this list. I tried to follow 
the code but got lost, otherwise I'd suggest a fix.

----------
components: Database
messages: 7612
nosy: rouilj, tekberg
severity: minor
status: new
title: config.ini doesn't list all available databases
type: behavior
versions: 2.2.0

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