Replace vcstools with vcstool

[esteve]

This PR is based on @cottsay work at https://github.com/ros-infrastructure/bloom/tree/cottsay%2Fvcstool, I've only rebased it and replaced a vcstools import.

[nuclearsandwich]

Thanks @esteve, this branch was still work-in-progress when last touched and was set aside in part because vcstool itself was not being actively maintained. We have a current Google Summer of Code project from @leander-dsouza to bring an updated vcstool to PyPI and the ROS apt repositories under a new name since we're unable to reach the current maintainer on PyPI and gain control of the project. One of the stretch goals for that project is to get bloom switched over to using it, but if you have interest in doing some of that work in parallel to that do feel free to add commits to this branch. For our part, we'll likely only start this work once https://github.com/ros-infrastructure/vcs2l is released on PyPI and the ROS bootstrap repository in a usable state.