Migrate to Python 3

[smola]

Ubuntu Trusty ships Python 3 by default, but not Python 2. Older versions already ship Python 3, so this change should be safe.

[FooBarWidget]

Wow, I'm surprised my_init didn't need any changes. I thought porting to Python 3 would take more time than just changing the shebang.

[smola]

Note that I haven't tested it that much yet. Just checked that it starts without errors.

[FooBarWidget]

Trusty isn't out yet, and Precise does not install Python 3 by default, so I'm putting this pull request on hold for now.

[jakedt]

I am using the released version of 14.04 and they have not migrated to Python 3 by default. I am still looking forward to a baseimage based on 14.04 asap since the versions of so many critical libraries have been updated quite a bit in the last two years. I personally need the newer python 2.7.6 and a newer version of phantomjs.

[kingdonb]

I have been using trusty for more than the last month in baseimage-docker and have found that this patch, as well as setuser with the same patch re: importing posixfile are pretty good. The only other thing that was needed, also was needed for saucy, --default-modules is no longer needed/appropriate on newer versions of syslog-ng.

You can check out my patches on branch baseimage-docker/saucy and the follow-up branch baseimage-docker/trusty which were cleanly rebased yesterday (but unfortunately have other changes not intended for a pull request). Now that there are upstream ubuntu:14.04 images tagged in docker, this can become a priority.

NB: I haven't tested the saucy branch, and if it's true that python3 is not installed by default, it probably won't work. But those changes are all good for trusty.

This is the change for image/setuser

[mpeterson]

This one has been included on #71

[FooBarWidget]

Let's move to PR #71.