fix / workaround potential for socket.getaddrinfo to raise socket.error

As reported in bug 1154599, I'm seeing this on my desktop system:
$ python -c \
  'from cloudinit import util; print util.is_resolvable("brickies.neiit")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "cloudinit/util.py", line 865, in is_resolvable
    socket.SOCK_STREAM, socket.AI_CANONNAME)