Fix plugin loading

Regression from d354cdd.
Fix `vagrant plugin` commands

`StateFile#installed_plugins` returns now a Hash instead of Array.

Fixes regression from 39b2539.
Merge pull request #2583 from tmatilai/plugin-regressions

commands/plugin: fix plugin regressions for new state file
move test/acceptance to test/acceptance_old
provisioners/docker: Ensure checks for whether the container exists / is running works accross multiple Docker versions [GH-2579]
provisioners/docker: Extract container lookup method
Merge pull request #2579 from mitchellh/2579-fix-docker-container-running-check

provisioners/docker: Check for whether the container is running is not working properly
Add missing argument to ProviderVirtualBox::SyncedFolder#driver call

GH-2577 missed passing the `machine` to the `driver` method.
Merge pull request #2586 from tmatilai/2577-synced-folder

provider/virtualbox: add missing argument to ProviderVirtualBox::SyncedFolder#driver call
fixed broken link
Merge pull request #2593 from bennlich/patch-1

fixed broken link on configuration version page
[provisioner/chef]: Update base to fix typo.

It seems this was introduced by accident with #2281.
Merge pull request #2594 from johnbellone/patch-1

provisioner/chef: Update base to fix typo.
Merge branch 'master' of github.com:mitchellh/vagrant
Start acceptance test config stuff
providers/virtualbox: only clear shared folders if we have a VM
test/acceptance: set the VBOX_USER_HOME
update rake tasks for acceptance testing
Update README
Update gemfile for others