Enable TLS server authentication, using the official exporter-toolkit

[tuannh99]

This enables TLS server authentication for kafka_exporter, using the official exporter-toolkit

level=info ts=2021-05-25T11:13:10.471Z caller=kafka_exporter.go:545 msg="Starting kafka_exporter" version="(version=, branch=, revision=)"
level=info ts=2021-05-25T11:13:10.472Z caller=kafka_exporter.go:546 build_context="(go=go1.16.3, user=, date=)"
level=info ts=2021-05-25T11:13:10.485Z caller=kafka_exporter.go:216 msg="Done Init Clients"
level=info ts=2021-05-25T11:13:10.486Z caller=kafka_exporter.go:688 msg="Listening on" address=:9308
level=info ts=2021-05-25T11:13:10.493Z caller=tls_config.go:224 msg="TLS is enabled." http2=true

[tuannh99]

@danielqsj Thoughts?