vagrant LXC support broken in Ubuntu Xenial

[Tim Abbott]

Vagrant 1.8.1 has a bug that results in this traceback when trying to use the vagrant-lxc plugin:

```
/usr/lib/ruby/2.3.0/rubygems/specification.rb:946:in `all=': undefined method `group_by' for nil:NilClass (NoMethodError)
  from /usr/lib/ruby/vendor_ruby/vagrant/bundler.rb:275:in `with_isolated_gem'
  from /usr/lib/ruby/vendor_ruby/vagrant/bundler.rb:231:in `internal_install'
  from /usr/lib/ruby/vendor_ruby/vagrant/bundler.rb:102:in `install'
  from /usr/lib/ruby/vendor_ruby/vagrant/plugin/manager.rb:62:in `block in install_plugin'
  from /usr/lib/ruby/vendor_ruby/vagrant/plugin/manager.rb:72:in `install_plugin'
  from /usr/share/vagrant/plugins/commands/plugin/action/install_gem.rb:37:in `call'
  from /usr/lib/ruby/vendor_ruby/vagrant/action/warden.rb:34:in `call'
  from /usr/lib/ruby/vendor_ruby/vagrant/action/builder.rb:116:in `call'
  from /usr/lib/ruby/vendor_ruby/vagrant/action/runner.rb:66:in `block in run'
  from /usr/lib/ruby/vendor_ruby/vagrant/util/busy.rb:19:in `busy'
  from /usr/lib/ruby/vendor_ruby/vagrant/action/runner.rb:66:in `run'
  from /usr/share/vagrant/plugins/commands/plugin/command/base.rb:14:in `action'
  from /usr/share/vagrant/plugins/commands/plugin/command/install.rb:32:in `block in execute'
  from /usr/share/vagrant/plugins/commands/plugin/command/install.rb:31:in `each'
  from /usr/share/vagrant/plugins/commands/plugin/command/install.rb:31:in `execute'
  from /usr/share/vagrant/plugins/commands/plugin/command/root.rb:56:in `execute'
  from /usr/lib/ruby/vendor_ruby/vagrant/cli.rb:42:in `execute'
  from /usr/lib/ruby/vendor_ruby/vagrant/environment.rb:268:in `cli'
  from /usr/bin/vagrant:173:in `<main>'
```

Reproducer:

```
sudo apt-get install git vagrant lxc lxc-templates cgroup-lite redir
vagrant plugin install vagrant-lxc
```

This bug was fixed in Vagrant 1.8.2; it could make sense to backport the full package or just the patch indicated in these links:

http://stackoverflow.com/questions/36811863/cant-install-vagrant-plugins-in-ubuntu-16-04/36991648#36991648

https://github.com/mitchellh/vagrant/issues/7073

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.