plugins https/ssl options inconsistencies

We should probably look into using a standard config option for using the `https`/`ssl` protocol. I did a little survey and this is what we currently use:
- *_protocol => ['http', 'https’] - logstash-output-elasticsearch-ec2
- ssl => boolean - logstash-output-elasticsearch
- ssl => boolean - logstash-input-rabbitmq
- proto => ['http', 'https’] - logstash-output-loggly
- ssl_enable = > boolean - logstash-input-tcp
- ssl => boolean - logstash-output-redmine
- enable_ssl => boolean - logstash-output-riak

I suggest we standardize on ssl:

``` ruby
config :ssl, :validate => :boolean
```