Fix failed LDAP logins when sync_ssh_keys is included in config

What does this MR do?

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/4662

We tell users to set sync_ssh_keys to false if they don't intend to use the feature. Strangely, true is not how you enable it, rather you give it an LDAP attribute name. The default value for sync_ssh_keys is nil which doesn't break Gitlab::LDAP::Person.ldap_attributes, but false does. This rejects false and true for good measure and adds an EE-only test to ensure it doesn't regress.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?