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.

- Replace custom code to set OF protocols with new Mininet available API to set.

- Updated for new Mininet 2.2.0
- Support TCP or SSL for remote controller
- Support HostWithPrivateDirs Hosts

Includes some big fixes and new feature to run user defined command during
node start and stop.

added persistence option to HostWithPrivateDirs. also attached mount namespaces when mnexec -a is specified

checking with non-numeric characters.

Add spaces.

Many changes since master version.
- save/load topologies
- host, controller, switch and link properties
- link actions to set status up/down