Resolve "Make the mapping between Kerberos and LDAP configurable"

What does this MR do?

This merge request introduces two new settings two make the mapping between Kerberos and LDAP more configurable as wished in #2906 This MR adds the possibility to enable simple_ldap_linking. If simple ldap linking is activated and someone tries to authenticate via Kerberos only two test have to succeed in order to successfully authenticate the user. Firstly the Kerberos username have to match ldap uid and secondly the Kerberos realm has to be listed in the simple_ldap_linking_allowed_realms setting.

What are the relevant issue numbers?

#2906

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer