Allow custom Spark UI Url

The motivation behind this improvement/change request is to have Zeppelin workaround known issues in Spark UI behind reverse proxy (see https://issues.apache.org/jira/browse/SPARK-20044 for example)

By adding a custom (zeppelin.spark.*) Spark parameter we override default Spark logic in building the URL path and redirect the Zeppelin user directly to a configured reverse proxy path.