Checkmk Werk 15554: user_config: allow all option for authorized_sites

ID: 15554 Title: user_config: allow all option for authorized_sites Component: REST API Level: 1 Class: Bug fix Version: 2.3.0b1 Prior to this werk, the "all" value in authorized_sites was rejected due to the validation mechanism. This werk fixes this issue. This fix does not modify the default behaviour of authorized_sites when creating an user.