buildbot/Dockerfile.template: Adjust Python 2 dependency

buildbot/Dockerfile.template: Adjust Python 2 dependency

Now that the ancestor gentoo-python image is removing Python 2, we
need to get it back for Buildbot [1,2].  We need to keep the
USE_PYTHON setting (with a tweaked value) because buildbot hasn't
transitioned to the *-r1 Python eclasses yet:

  $ grep 'inherit.*distutils' /usr/portage/dev-util/buildbot*/*ebuild
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-0.8.7_p1.ebuild:inherit distutils readme.gentoo user
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-0.8.8-r1.ebuild:inherit distutils readme.gentoo systemd user
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-0.8.8.ebuild:inherit distutils readme.gentoo user
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-0.8.9.ebuild:inherit distutils readme.gentoo systemd user
  /usr/portage/dev-util/buildbot-slave/buildbot-slave-9999.ebuild:inherit distutils readme.gentoo user
  /usr/portage/dev-util/buildbot/buildbot-0.8.7_p1-r1.ebuild:inherit distutils readme.gentoo user
  /usr/portage/dev-util/buildbot/buildbot-0.8.8-r2.ebuild:inherit distutils readme.gentoo systemd user
  /usr/portage/dev-util/buildbot/buildbot-0.8.9.ebuild:inherit distutils readme.gentoo systemd user
  /usr/portage/dev-util/buildbot/buildbot-9999.ebuild:inherit distutils readme.gentoo user systemd

Also, instead of hardcoding the packages effected by the sqlite USE
flag change, just use --newuse on @system and @world and let Portage
figure out what needs to be rebuilt.

[1]: Buildbot will transition after Twisted
     (http://trac.buildbot.net/wiki/CompatibilityPolicy#Python3)
[2]: Twisted is 60% of the way there (94/157 Python 3 tickets,
     https://twistedmatrix.com/trac/milestone/Python-3.x)