skip pip dependency in bloom-generate with --skip-pip

[k-okada]

this PR should fix the problem on some of jsk packages,

The following packages have unmet dependencies:
 ros-one-jsk-data : Depends: gdown but it is not installable
 ros-one-jsk-tools : Depends: pygithub3 but it is not installable
                     Depends: python3-rosdep but it is not installable
                     Depends: slacker-cli but it is not installable
                     Depends: tabulate but it is not installable
E: Unable to correct problems, you have held broken packages.
apt-get failed.

To use this, we need ros-infrastructure/bloom#412, and I don't know whether to put this code in build.sh or prepare.sh.

[k-okada]

resoved conflict

[v4hn]

Thanks for pointing me to your --skip-pip patch, I didn't know about it.
Your group clearly spent a lot of work on getting the pip dependencies supported on some level.

I'm not really happy with the flag though, for the same reasons discussed in the pull-request.
For now, I decided to forego these dependencies altogether and explicitly map them to an empty set of packages.
It's not an ideal solution either because the meta-data is lost, but having dependencies between different package distributors was a bad idea in the first place in my opinion and leaving the user to run rosdep to install things via pip is problematic as well.
I very much see where the need for it is coming from though and it got much worse since 2016 with all the transformer code and pip bundles that will randomly segfault if you build them yourself...
In general, I believe catkin_virtualenv is a much better solution for getting pip involved, but it needs more changes to map it to system debs at some point in the future, and also comes with its own problems.