Split IPClusterApp into multiple Application subclasses for each subcommand

[Brian Granger]

IPClusterApp has multiple subcommand (start, list, stop, create). All of these are currently implemented in a single Application subclass. We need to create a common base class that has dispatching to split the different subcommand into their own Application subclasses.

[Brian Granger (ellisonbg)]

This would help resolve some of the difficulties we have in finding and creating cluster directories. If the user gives the --cluster-dir option to specify a cluster_dir, it currrently looks in cwd, but only proceeds if the directory already exists. That is fine for start/stop, but awkward for create as it requires the user to first create the directory using mkdir and then to ipcluster create. Basically, create needs to handle finding the cluster dir differently than start/stop. What is also awkward is that start can also auto create the cluster dir, but it should do so differently than create. But what if ipcluster start is run with the --cluster-dir option?

Splitting IPClusterApp into different sub apps will enable us to better utilize custom logic for each app.