[ Reason ]
At installation time of the Debian Edu mainserver, the Debian
Installer tries to configure icinga2-ido-mysql. This must be avoided,
the configuration of the package is handled by specific Debian Edu
scripts later in the installation process.

[ Impact ]
Debian Edu main server will continue to fail during installation.

[ Tests ]
Manual introspection of the code, test with Debian Edu sid image.

[ Risks ]
Debian Edu only.

[ Checklist ]
  [x] *all* changes are documented in the d/changelog
  [x] I reviewed all changes and I approve them
  [x] attach debdiff against the package in (old)stable
  [x] the issue is verified as fixed in unstable

[ Changes ]

+  * preseed-values/defaults.main-server: Preseed
+    icinga2-ido-mysql/dbconfig-install to false. Needed after
+    debian-edu 2.13.1 (d5c688a7) and debian-edu-config 858a9689.

[ Other info ]
None.