Add support for custom truststore to Knox shell client

The Knox shell client does not verify the certificate of the server.

One option would be to provide another method where developers can provide their own client, e.g.

public static Hadoop login( String url, String username, String password, HttpClient client ) throws URISyntaxException { }

https://github.com/apache/knox/blob/master/gateway-shell/src/main/java/org/apache/hadoop/gateway/shell/Hadoop.java#L60

I can provide a patch if you are happy with this approach.