Allow user override of rosdep source lists
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

georgepar/rosdep,wkentaro/rosdep,ros-infrastructure/rosdep,alessandro-aglietti/rosdep,sorki/rosdep,aymanim/rosdep,allenh1/rosdep,allenh1/rosdep,aymanim/rosdep,spaghetti-/rosdep,alessandro-aglietti/rosdep,sorki/rosdep,georgepar/rosdep,spaghetti-/rosdep,ros-infrastructure/rosdep,wkentaro/rosdep