3856: Enable configuring trusted certificates for Active Directory setup

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What version are you running?

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Describe the enhancement and the motivation for it.
Active Directory can be connected via TLS, which increases security. This is already supported, but our organization uses custom intenal CA to issue certificate for AD domain controllers. This CA is not trusted by default, so after enabling TLS, all authentication attempts fail with the following message in the server log: 'Active Directory: Could not connect to domain controller <dc.name>:389 for domain <domain>, possibly the certificate wasn't verifiable'.
Please allow setting up custom trusted certificates for the purpose of TLS use.

What operating system are you using? What browser?
Windows 8.1, Firefox DeveloperEdition 39.0a2

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