maas builds fail when distro-info-data runs out of data

We hit failures building xenial images during the window of time when the zesty entry in distro-info-data expired and before the package was updated with artful. While it's expected that building a not-yet-named release would fail, this should not impact xenial builds.

Here's the relevant stack trace from the build log:

Building images for v2 streams
+ PATH=/build/config/maas/bin:/build/local/scripts:/build/local/scripts:/build/local/scripts:/build/local/scripts:/build/local/scripts:/build/local/scripts:/build/local/scripts:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin
+ meph2-build --image-format root-tar -vv --enable-di amd64 xenial 20170420 livecd.ubuntu-cpc.rootfs.tar.xz binary/maas/
Traceback (most recent call last):
  File "/build/config/maas/bin/meph2-build", line 10, in call_entry_point
    __import__(istr)
ImportError: No module named 'meph2'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/build/config/maas/meph2/ubuntu_info.py", line 97, in get_ubuntu_info
    devel = udi.devel(date=date)
  File "/usr/lib/python3/dist-packages/distro_info.py", line 96, in devel
    raise DistroDataOutdated()
distro_info.DistroDataOutdated: Distribution data outdated. Please check for an update for distro-info-data. See /usr/share/doc/distro-info-data/README.Debian for details.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/build/config/maas/bin/meph2-build", line 25, in <module>
    call_entry_point("meph2.commands.build_image.main")
  File "/build/config/maas/bin/meph2-build", line 17, in call_entry_point
    __import__(istr)
  File "/build/config/maas/meph2/commands/build_image.py", line 7, in <module>
    from meph2.stream import (
  File "/build/config/maas/meph2/stream.py", line 2, in <module>
    from . netinst import (POCKETS, POCKETS_PROPOSED, get_di_kernelinfo,
  File "/build/config/maas/meph2/netinst.py", line 24, in <module>
    from .ubuntu_info import REL2VER, SUPPORTED
  File "/build/config/maas/meph2/ubuntu_info.py", line 131, in <module>
    info = get_ubuntu_info()
  File "/build/config/maas/meph2/ubuntu_info.py", line 99, in get_ubuntu_info
    sys.stderr.write(
NameError: name 'sys' is not defined

The problem may just be the missing "import sys" in this code path.