Add support for yakkety

[Dan Watkins]

When building yakkety cloud images, we hit an error when trying to generate the MAAS images:

+ meph2-build --image-format root-tar -vv --enable-di ppc64el yakkety 20160425 livecd.ubuntu-cpc.rootfs.tar.xz binary/maas/
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 22, in call_entry_point
    sys.exit(getattr(sys.modules[istr], ent)())
  File "/build/config/maas/meph2/commands/build_image.py", line 101, in main
    verbosity=vlevel, img_format=args.image_format)
  File "/build/config/maas/meph2/stream.py", line 77, in create_version
    version = rdata['version']
TypeError: 'NoneType' object is not subscriptable

It looks like this is caused by there being no definitions for yakkety in conf/meph-v2.yaml. Please address this!