[ZEPPELIN-4393] Support for '--' comment in Cassandra interpreter

[alexott]

What is this PR for?

Cassandra Query Language (CQL) supports 2 types of end-of-line comments - // and --, but Cassandra interpreter supports only first one, resulting in error when using -- comment in the query.

What type of PR is it?

Bug

What is the Jira issue?

• https://issues.apache.org/jira/browse/ZEPPELIN-4393

How should this be tested?

• 2 unit tests were added - for both comments types
• Tested manually
• Travis build: https://travis-ci.org/alexott/zeppelin/builds/603532433

Questions:

• Does the licenses files need update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? Documentation was updated

[zjffdu]

Thanks @alexott LGTM