Rename docker_client and docker_installer

We can rename those classes and remove the docker_ prefix as they
already have a module namespace.
website/docs: phrasal verbs [GH-2644]
core: box removal of a V1 box works
Merge pull request #2641 from arthurnn/docker_rename

Rename docker_client and docker_installer
Update CHANGELOG
provisioners/chef: fix node/client deletion when node_name is not set [GH-2345]

V2 config uses `hostname` instead of `host_name`.
Add Vagrant command plugin for snapshot management.

I see that there are a lot of different gems for this. I found myself
writing yet another for OpenStack where it makes sense to implement it
for the default provider (VirtualBox) and then let the other provider
plugins implement it if necessary.
Update commands to add restore option.

This also changes some of the terminology to follow the snapshot
commands in VirtualBox. I am not sure if its better to stick with 'take'
and 'destroy' or 'take' and 'delete', etc.
Add implementation of snapshots with VirtualBox.
Wire up create snapshot actions with VirtualBox.

This wires up the create action/command with the VirtualBox provider driver.
Wire up delete snapshot actions with VirtualBox.
Wire up list snapshot actions with VirtualBox.
Write up restore snapshot actions with Virtualbox.