Set u+w perms on private keys in tmp dir.

This should allow additional `vagrant provision` calls to work when
using the salt provisioner and providing private keys. See #3542.
Merge pull request #3575 from shadowfax-chc/salt-provision-private-key-perms

provisioner/salt: Set u+w perms on private keys in tmp dir.
Update CHANGELOG.md
website/www: feature preview docker provider
providers/docker: raise better error for local executor
providers/docker: clearer options on docker-logs
providers/docker: pty flag, not functional yet
add dependency on Ruby >= 2.0.0

Vagrant requires Ruby 2, e.g. for ** keyword arguments.
providers/docker: use ssh_run if asking for stdin on execute
providers/docker: with a pty, single target the docker-run cmd
Merge pull request #3580 from mitchellh/f-tty-docker-run

vagrant docker-run tty
Removing some old files
Handle a lot of TODOs
core: Lock in plugin install of a specific version
Fix tests
Merge pull request #3581 from aspiers/require-ruby-2.x

add dependency on Ruby >= 2.0.0
backport GH-3277 fix for missing interpolation to older drivers

The original fix for the "missing interpolation argument :stderr" bug
was in d4b3f226, but it only addressed the driver for VirtualBox 4.3:

  https://github.com/mitchellh/vagrant/issues/3277

This backports the fix to the drivers for VirtualBox 4.0, 4.1, and 4.2.
providers/docker: use the proper class
Merge pull request #3585 from aspiers/backport-3277-old-virtualboxen

backport GH-3277 fix for missing interpolation to older drivers
Update README