different default values are used for parameter 'dbtype`

[j-ed]

refers to #3069

Expected behavior

For all parameters default values should be set and the same default value should be used for all appearances in the source code.

Current behavior

Not all of the parameters are set to a fixed default value, nor have they the same default value assigned in the source code.

As far as I understood the default value of the parameter dbtype should be sqlite but other values have been assigned in the source code:

+ getSystemValue('dbtype', 'sqlite')                 # default value !!
- /var/www/htdocs/nextcloud/lib/private/Setup/AbstractDatabase.php:140
- /var/www/htdocs/nextcloud/lib/private/AllConfig.php:438
- /var/www/htdocs/nextcloud/lib/private/App/Platform.php:72
- /var/www/htdocs/nextcloud/lib/private/DB/MDB2SchemaWriter.php:46
- /var/www/htdocs/nextcloud/lib/private/DB/MDB2SchemaWriter.php:53
- /var/www/htdocs/nextcloud/lib/private/Settings/Admin/TipsTricks.php:45

+ getSystemValue('dbtype', '')                       # <<< 'sqlite' missing
  - /var/www/htdocs/nextcloud/core/Command/Db/ConvertType.php:132

+ getSystemValue('dbtype')                           # <<< 'sqlite' missing
  - /var/www/htdocs/nextcloud/apps/survey_client/lib/Categories/Database.php:74
  - /var/www/htdocs/nextcloud/apps/survey_client/lib/Categories/Database.php:81:
  - /var/www/htdocs/nextcloud/apps/survey_client/lib/Categories/Database.php:122
  - /var/www/htdocs/nextcloud/apps/serverinfo/lib/DatabaseStatistics.php:51
  - /var/www/htdocs/nextcloud/apps/serverinfo/lib/DatabaseStatistics.php:58
  - /var/www/htdocs/nextcloud/apps/serverinfo/lib/DatabaseStatistics.php:93

Steps to reproduce

1. search and compare the assigned values in the getSystemValue() call.

Environment

Server Configuration

OS: Linux 3.2.82
Web server: Apache2 2.4.25
Database: MariaDB 5.5.53
PHP version: 5.6.29
Nextcloud version: 11.0.0

Client Configuration

Browser: Mozilla Firefox 50.1.0
Operating system: Windows 7

[MorrisJobke]

That's okay. Only the setup calls need to proper default value. This is a mandatory setting and a fallback to the default can be better detected as failure if it is a not allowed value.