[cosmetic] remove trailing whitespaces in sources_list.py
Allow user override of rosdep source lists

ROSDEP_SOURCES_PATH should be a colon-separated list of directories
If specified, it will replace the system default location for sources:
 '/etc/ros/rosdep/sources.list.d'

The order matters: all files from each item in the PATH will have
precedence over all files in the next items.

Closes #337
Merge pull request #342 from po1/user-sources

Allow user override of rosdep source lists