[SPARK-8528] Expose SparkContext.applicationId in PySpark

Use case - we want to log applicationId (YARN in hour case) to request help with troubleshooting from the DevOps

Author: Vladimir Vladimirov <vladimir.vladimirov@magnetic.com>

Closes #6936 from smartkiwi/master and squashes the following commits:

870338b [Vladimir Vladimirov] this would make doctest to run in python3
0eae619 [Vladimir Vladimirov] Scala doesn't use u'...' for unicode literals
14d77a8 [Vladimir Vladimirov] stop using ELLIPSIS
b4ebfc5 [Vladimir Vladimirov] addressed PR feedback - updated docstring
223a32f [Vladimir Vladimirov] fixed test - applicationId is property that returns the string
3221f5a [Vladimir Vladimirov] [SPARK-8528] added documentation for Scala
2cff090 [Vladimir Vladimirov] [SPARK-8528] add applicationId property for SparkContext object in pyspark