accept `self_signed_ssl` option

Proposed solution for issue #4, which was also referenced in [cf-services-contrib-release#81](https://github.com/cloudfoundry/cf-services-contrib-release/pull/81)

I want to be able to disable `Excon.defaults[:ssl_verify_peer]` when I connect to Swift using self signed certificates. For this I need to be able to pass in the option `:self_signed_ssl`. This is further explained in the issue filed by @rkoster. PR will be followed up with another one at cf-services-contrib-release, making it possible to include the option in your bosh manifest.