added parameter default values

based on a source code analysis (looking for getSystemValue() function) I added the default values of the prameters

Update config.sample.php

replaced three backticks by two.

applied various fixes

- removed default value for 'objectstore' because it has not default value.
- removed default value for 'tempdirectory' because it is unset.
- changed default value for 'theme' to "Defaults to the theming app which is shipped since Nextcloud 9"
- fixed typo in default value of 'minimum.supported.desktop.version', after double-checking the version
  number in /apps/dav/lib/Connector/Sabre/BlockLegacyClientPlugin.php:71.

Signed-off-by: Juergen Edner <juergen@eisfair.org>