Default auth mechanism

[bobapple]

The user should be able to set a default authentication mechanism when declaring ::icingaweb2 (db, ldap etc.) which then gets configured and initialised automatically if possible. Dependencies of the chosen auth method should be installed if manage_package is set to true

We need this to make it more comfortable for the user to use the module. He should not need to create the resources manually.

[bobapple]

A default user with password is created during the import_schema process.