Former ldap user can not login

Created by: jcockhren

When a user has ldap credentials, the system attempts to access the ldap server even if the ldap authentication is disabled in the config. The following error appears:

Net::LDAP::LdapError (No such address or other socket error.):
  lib/gitlab/ldap/adapter.rb:7:in `open'
  lib/gitlab/ldap/access.rb:7:in `open'
  app/controllers/application_controller.rb:203:in `gitlab_ldap_access'
  app/controllers/application_controller.rb:184:in `ldap_security_check'

This issue existed as #6188 (closed) before the merge with the gitlab EE codebase and was resolved with #6189.

Given the merge of the codebases, the issue resurfaced given the difference in approaches to authentication.