Fix LDAP tls_options not working

What does this MR do and why

The removal of the Settingslogic gem in !113040 (merged) created a regression when LDAP tls_options were used. It resulted in Gitlab::config.ldap.servers.main.tls_options.cert returning nil instead of the certificate.

This occurred because Gitlab::Auth::Ldap::Config#custom_tls_options attempts to duplicate the tls_options Hash and symbolize the keys, but instead it altered the internal Hash representation of GitlabSettings::Options. Since all the keys were transformed to symbols but GitlabSettings::Options converts all keys to strings, the values were inaccessible.

To fix this, implement a dup to create a copy of the internal representation. We should also consider refactoring Gitlab::Auth::Ldap::Config.

Relates to #413017 (closed)

How to set up and validate locally

1. Use the master branch.
2. In your config/gitlab.yml, use this block (you may have to remove the exising ldap line):

  ldap:
    enabled: true
    servers:
      main: {"label":"Google Secure LDAP","host":"ldap.google.com","port":636,"uid":"uid","bind_dn":"XXXX","password":"XXXXX","encryption":"simple_tls","verify_certificates":false,"retry_empty_result_with_codes":[80\
],"base":"DC=XX,DC=XX","tls_options":{"cert":"-----BEGIN CERTIFICATE-----\nXXXX\n-----END CERTIFICATE-----\n","key":"-----BEGIN PRIVATE KEY-----\nXXXXX\n-----END PRIVATE KEY-----\n"}}

3. Run bin/rails console
4. Enter: Gitlab::config.ldap.servers.main.tls_options.cert. You should get a nil value back.
5. Change to this branch and repeat steps 3 and 4.

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