Changes to Vagrant provisioner and environment to accommodate new containerisation architecture.

Major changes include:

* Provisioning is done entirely with `vagrant up`. This has not been
easy and feels inelegant. Vagrant modifies the GEM_PATH env variable
meaning that it has to be saved and injected into subsequent `knife`
commands.

* Vagrantfile.local's symlinking magic is no longer needed

* DB reset now uses JSON fixtures rather than SQL.

* New ./dshell script for conveniently getting a terminal in a
container.

* There is an accompanying change in the cookbook which causes the
docker daemon service to delay its init until after the upstart
'vagrant-mounted' event has been triggered.