[SPARK-16796][WEB UI] Visible passwords on Spark environment page

## What changes were proposed in this pull request?

Mask spark.ssl.keyPassword, spark.ssl.keyStorePassword, spark.ssl.trustStorePassword in Web UI environment page.
(Changes their values to ***** in env. page)

## How was this patch tested?

I've built spark, run spark shell and checked that this values have been masked with *****.

Also run tests:
./dev/run-tests

[info] ScalaTest
[info] Run completed in 1 hour, 9 minutes, 5 seconds.
[info] Total number of tests run: 2166
[info] Suites: completed 65, aborted 0
[info] Tests: succeeded 2166, failed 0, canceled 0, ignored 590, pending 0
[info] All tests passed.

![mask](https://cloud.githubusercontent.com/assets/15244468/17262154/7641e132-55e2-11e6-8a6c-30ead77c7372.png)

Author: Artur Sukhenko <artur.sukhenko@gmail.com>

Closes #14409 from Devian-ua/maskpass.