Fix failed LDAP logins when sync_ssh_keys is included in config

Merged Drew Blessing requested to merge dblessing/gitlab:fix_ldap_sync_ssh_keys_boolean into master

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?

What are the relevant issue numbers?

Edited by Stan Hu