crypto/tls.ClientConfig: Set Config.Certificates for backwards compat

[aknuds1]

What this PR does:
A recent change in crypto/tls.ClientConfig.GetTLSConfig that allows for reloading of client certs, turned out to break memberlist/kv.NewTCPTransport. Apparently the latter uses ClientConfig for a server, which then fails since there's no longer any certificate config for servers.

I propose working around the problem by again setting crypto/tls.Config.Certificates, as it used to be prior to the aforementioned change. However, since we now also set GetClientCertificate, clients will prefer the latter:

// GetClientCertificate, if not nil, is called when a server requests a
// certificate from a client. If set, the contents of Certificates will
// be ignored.

I've tested that this change fixes the broken integration test in Mimir (TestSingleBinaryWithMemberlist/tls).

Which issue(s) this PR fixes:

Checklist

• Tests updated
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[zalegrala]

Looks good to me.