Unfortunately disabling IPv6 via sysctl doesn't actually
disable it on all of the interfaces by default. Disabling
it via grub disables it entirely in the VM.

Helps with #454

This should fix #398 for real; note that if we try to shut down ntpd
right at boot, it doesn't work! ;-(

However, setting the Unix time in the traditional manner using seconds
since 1970 should do the trick!

This should fix the problem where we see the first test
taking negative time, as well as possibly other issues
with performance tests which may be sensitive to changes
in wall clock time.

Fixes #398

This should help with #393 although it doesn't solve
the root issue of the example failing silently when
vconfig is missing.

This should more reliably shut down pexpect subprocesses when
build.py exits before completion.

Apparently there is a conflict where the lua plugin
conflicts with the built-in dissector for openflow
that is included with 1.12 and up. For now, we will
just not install the plugin. This should fix the
14.10 VM build.

Additionally, I have added a handy function,
version_ge, which compares version numbers in
canonical x.y.z format. Thanks to stackoverflow for
pointing out the incredibly useful sort -V option!

The older wireshark dissectors were not well-maintained
and were a pain to build. They also added tons of extra junk
into our VM images! The ones built into the current
wireshark are deficient for 1.3. The solution for the
future is almost certainly to go with the lua-based plugin,
since it is architecture-independent and should be much
easier to maintain and upgrade.

We add a new experimental feature to allow Mininet to run across
a cluster of machines. This is currently implemented via a set
mix-in classes that provide remote nodes that are implemented
via a connection to a remote shell, and remote links which are
tunnels across servers. In this preliminary implementation,
both control and data connections are made via ssh, but this
could change in the future.

A MininetCluster class is provided which allows existing code
to be used with minimal modification - all that is required is
to provide a list of servers to use. A customizable placement
algorithm may also be specified. An experimental CLI subclass
is also provided to make it easier to examine node placement;
status and links commands can also check whether nodes and
tunnels are still running.

Although this is an experimental feature, it does include a
--cluster option to make it convenient to start up a Mininet
simulation over a cluster, and a script to assist with setting
up the prerequisite authentication via ssh key pairs.

The cluster feature is preliminary and missing some obvious
important features, such as parallel startup and multiple tunnel
types, which we hope to add in the future.

file no longer returns image size on 14.04
fixes #373