Modularize Rake tasks

The top-level Rakefile now loads `tasks/*.rake` which allows the debug kit to
be extended with new rake tasks.

Closes puppetlabs/puppet-debugging-kit#3.
Rake can now list defined boxes

The task `rake list:boxes` will print out all custom boxes available to the
debugging kit.

Closes puppetlabs/puppet-debugging-kit#8.
Update PE 3.3.x VMs to 3.3.1
Clean up example VMs

Removed all PE VMs except for the latest 2.7.x and 3.x releases. This cuts down
on clutter for new users who don't necessarily need older version of PE.

Renamed `poss-head-centos` to `poss-head-master`.