[SPARK-2067] use relative path for Spark logo in UI

Author: Neville Li <neville@spotify.com>

Closes #1006 from nevillelyh/gh/SPARK-2067 and squashes the following commits:

9ee64cf [Neville Li] [SPARK-2067] use relative path for Spark logo in UI
(cherry picked from commit 15ddbef414d5fd6d4672936ba3c747b5fb7ab52b)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>