`vagrant ssh` and `vagrant ssh -c` are broken on master!

The problem seems to be related to recent changes that allowed multiple private keys. I suspect that the commit that introduced this was 45e09eb6778fd22d7d6cabae65f476d9f8035873

```
$ be vagrant ssh
You appear to be running Vagrant in a Bundler environment. Because
Vagrant should be run within installers (outside of Bundler), Vagrant
will assume that you're developing plugins and will change its behavior
in certain ways to better assist plugin development.

/home/projects/oss/vagrant/lib/vagrant/action/builtin/ssh_exec.rb:33:in `initialize': no implicit conversion of Array into String (TypeError)
	from /home/projects/oss/vagrant/lib/vagrant/action/builtin/ssh_exec.rb:33:in `new'
	from /home/projects/oss/vagrant/lib/vagrant/action/builtin/ssh_exec.rb:33:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/warden.rb:34:in `call'
	from /home/fabio/.rvm/gems/ruby-2.0.0-p247/bundler/gems/vagrant-lxc-c7fff4d9354b/lib/vagrant-lxc/action/check_running.rb:16:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/warden.rb:34:in `call'
	from /home/fabio/.rvm/gems/ruby-2.0.0-p247/bundler/gems/vagrant-lxc-c7fff4d9354b/lib/vagrant-lxc/action/check_created.rb:16:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/warden.rb:34:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/builder.rb:116:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/runner.rb:62:in `block in run'
	from /home/projects/oss/vagrant/lib/vagrant/util/busy.rb:19:in `busy'
	from /home/projects/oss/vagrant/lib/vagrant/action/runner.rb:62:in `run'
	from /home/projects/oss/vagrant/lib/vagrant/machine.rb:147:in `action'
	from /home/projects/oss/vagrant/plugins/commands/ssh/command.rb:56:in `block in execute'
	from /home/projects/oss/vagrant/lib/vagrant/plugin/v2/command.rb:193:in `block in with_target_vms'
	from /home/projects/oss/vagrant/lib/vagrant/plugin/v2/command.rb:191:in `each'
	from /home/projects/oss/vagrant/lib/vagrant/plugin/v2/command.rb:191:in `with_target_vms'
	from /home/projects/oss/vagrant/plugins/commands/ssh/command.rb:40:in `execute'
	from /home/projects/oss/vagrant/lib/vagrant/cli.rb:38:in `execute'
	from /home/projects/oss/vagrant/lib/vagrant/environment.rb:484:in `cli'
	from /home/projects/oss/vagrant/bin/vagrant:127:in `<top (required)>'
	from /home/fabio/.rvm/gems/ruby-2.0.0-p247/bin/vagrant:23:in `load'
	from /home/fabio/.rvm/gems/ruby-2.0.0-p247/bin/vagrant:23:in `<main>'

$ be vagrant ssh -c 'ls -la'
You appear to be running Vagrant in a Bundler environment. Because
Vagrant should be run within installers (outside of Bundler), Vagrant
will assume that you're developing plugins and will change its behavior
in certain ways to better assist plugin development.

/home/projects/oss/vagrant/lib/vagrant/action/builtin/ssh_run.rb:27:in `initialize': no implicit conversion of Array into String (TypeError)
	from /home/projects/oss/vagrant/lib/vagrant/action/builtin/ssh_run.rb:27:in `new'
	from /home/projects/oss/vagrant/lib/vagrant/action/builtin/ssh_run.rb:27:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/warden.rb:34:in `call'
	from /home/fabio/.rvm/gems/ruby-2.0.0-p247/bundler/gems/vagrant-lxc-c7fff4d9354b/lib/vagrant-lxc/action/check_running.rb:16:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/warden.rb:34:in `call'
	from /home/fabio/.rvm/gems/ruby-2.0.0-p247/bundler/gems/vagrant-lxc-c7fff4d9354b/lib/vagrant-lxc/action/check_created.rb:16:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/warden.rb:34:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/builder.rb:116:in `call'
	from /home/projects/oss/vagrant/lib/vagrant/action/runner.rb:62:in `block in run'
	from /home/projects/oss/vagrant/lib/vagrant/util/busy.rb:19:in `busy'
	from /home/projects/oss/vagrant/lib/vagrant/action/runner.rb:62:in `run'
	from /home/projects/oss/vagrant/lib/vagrant/machine.rb:147:in `action'
	from /home/projects/oss/vagrant/plugins/commands/ssh/command.rb:43:in `block in execute'
	from /home/projects/oss/vagrant/lib/vagrant/plugin/v2/command.rb:193:in `block in with_target_vms'
	from /home/projects/oss/vagrant/lib/vagrant/plugin/v2/command.rb:191:in `each'
	from /home/projects/oss/vagrant/lib/vagrant/plugin/v2/command.rb:191:in `with_target_vms'
	from /home/projects/oss/vagrant/plugins/commands/ssh/command.rb:40:in `execute'
	from /home/projects/oss/vagrant/lib/vagrant/cli.rb:38:in `execute'
	from /home/projects/oss/vagrant/lib/vagrant/environment.rb:484:in `cli'
	from /home/projects/oss/vagrant/bin/vagrant:127:in `<top (required)>'
	from /home/fabio/.rvm/gems/ruby-2.0.0-p247/bin/vagrant:23:in `load'
	from /home/fabio/.rvm/gems/ruby-2.0.0-p247/bin/vagrant:23:in `<main>'
```