Support TLS for self signed certificates

[radekg]

Hi @jamiealquiza. This pull request adds support for TLS with self signed certificates. In case of a self signed certificate, there will be no root ca (and thus no matching file). The new flags -tls-cert-file and -tls-key-file, together with -tls-insecure-skip-verify, allow for self-signed certificate usage.

Example:

$GOPATH/bin/sangrenel \
    -brokers=... \
    -message-batch-size=500 \
    -message-size=500 \
    -required-acks=all \
    -topic=scratch.kafka-proxy-test.dshtest \
     -tls \
     -tls-cert-file=$HOME/cert.pem \
     -tls-key-file=$HOME/key.pem \
     -tls-insecure-skip-verify

[jamiealquiza]

looks good, merging

[radekg]

Wow. Cool, that was fast. Thank you!