[SPARK-8429] [EC2] Add ability to set additional tags

Add the `--additional-tags` parameter that allows to set additional tags to all the created instances (masters and slaves).

The user can specify multiple tags by separating them with a comma (`,`), while each tag name and value should be separated by a colon (`:`); for example, `Task:MySparkProject,Env:production` would add two tags, `Task` and `Env`, with the given values.

Author: Stefano Parmesan <s.parmesan@gmail.com>

Closes #6857 from armisael/patch-1 and squashes the following commits:

c5ac92c [Stefano Parmesan] python style (pep8)
8e614f1 [Stefano Parmesan] Set multiple tags in a single request
bfc56af [Stefano Parmesan] Address SPARK-7900 by inceasing sleep time
daf8615 [Stefano Parmesan] Add ability to set additional tags