Add AMQPS TLS config support to AMQP plugins

Adds an optional `tls` subsection to AMQP plugins much akin to other inputs and outputs that support TLS. This is useful e.g. if your AMQP broker is set to verify client certificates with `{verify,verify_peer}` and `{fail_if_no_peer_cert,true}` and as such Heka needs to present a client certificate.