[SPARK-9572] [STREAMING] [PYSPARK] Added StreamingContext.getActiveOrCreate() in Python

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #8080 from tdas/SPARK-9572 and squashes the following commits:

64a231d [Tathagata Das] Fix based on comments
741a0d0 [Tathagata Das] Fixed style
f4f094c [Tathagata Das] Tweaked test
9afcdbe [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-9572
e21488d [Tathagata Das] Minor update
1a371d9 [Tathagata Das] Addressed comments.
60479da [Tathagata Das] Fixed indent
9c2da9c [Tathagata Das] Fixed bugs
b5bd32c [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into SPARK-9572
b55b348 [Tathagata Das] Removed prints
5781728 [Tathagata Das] Fix style issues
b711214 [Tathagata Das] Reverted run-tests.py
643b59d [Tathagata Das] Revert unnecessary change
150e58c [Tathagata Das] Added StreamingContext.getActiveOrCreate() in Python