Rosdep yaml file cleaning

Cleaning enforces the following rules:
 * no trailing spaces
 * indentation is 2 spaces
 * lists of packages are in bracket [] form
 * scripts are written with the YAML | block syntax
 * items are listed in alphabetical order

A script has been provided: scripts/clean_rosdep_yaml.py
This script takes any valid YAML file and will clean it
consistently with the rules above.