add the ability to load an ssl certificate from a file
add the ability to load an ssl certificate from a file

This is needed so that we can support multiple PEM encoded certs in the
same file, as ruby's openssl bindings do not allow you to parse PEMs in
any other way (unless you write your own parser)

zendesk/ruby-kafka,crashlytics/ruby-kafka,crashlytics/ruby-kafka,zendesk/ruby-kafka