Add SSL option

I've added support for basic for SSL connections to Zeppelin. This might be useful for the cases when communication with the Zeppelin server needs to happen over the internet. Setting Zeppelin up to use SSL requires enabling and changing the corresponding attributes in zeppelin-site.xml. The necessary files for server-only authenticated connections require a keystore. Using client-authenticated connections also require a truststore. Keytool and OpenSSL can be used to generate these objects. Reference here: https://wiki.eclipse.org/Jetty/Howto/Configure_SSL.

If SSL option is not enabled, everything should work just fine. As a nice-have add-on the Websocket port can be set inside the zeppelin-site.xml as well. This might be helpful to select commonly used SSL ports numbers that are not exactly 1 apart.