Remove supports_attach? and call to /sbin/halt

Hosts without support for lxc-attach are ancien and can always use old
versions of vagrant-lxc. To be able to move forward more easily, we
should be able to assume a functional `lxc-attach`.

As for `/sbin/halt`, I'm really not sure it's needed anymore. Let's see
if its removal causes problems.

lib/vagrant-lxc/driver.rb

@@ -122,10 +122,6 @@ module Vagrant
         @cli.destroy
       end
 
-      def supports_attach?
-        @cli.supports_attach?
-      end
-
       def attach(*command)
         @cli.attach(*command)
       end

lib/vagrant-lxc/driver/cli.rb

@@ -89,7 +89,6 @@ module Vagrant
         # Man Page:
         # 2      The specified container exists but was not running.
         def stop
-          attach '/sbin/halt' if supports_attach?
           begin
             run :stop, '--name', @name
           rescue LXC::Errors::ExecuteError => e
@@ -142,19 +141,6 @@ module Vagrant
           end
         end
 
-        def supports_attach?
-          unless defined?(@supports_attach)
-            begin
-              @supports_attach = true
-              run(:attach, '--name', @name, '--', '/bin/true')
-            rescue LXC::Errors::ExecuteError
-              @supports_attach = false
-            end
-          end
-
-          return @supports_attach
-        end
-
         private
 
         def run(command, *args)

spec/unit/driver/cli_spec.rb

@@ -134,39 +134,14 @@ describe Vagrant::LXC::Driver::CLI do
 
     before do
       allow(subject).to receive(:run)
-    end
-
-    context 'lxc-attach is supported' do
-      before do
-        subject.stub(attach: true, supports_attach?: true)
       subject.stop
     end
 
-      it 'runs a /sbin/halt within the container' do
-        expect(subject).to have_received(:attach).with('/sbin/halt')
-      end
-
     it 'issues a lxc-stop with provided container name' do
       expect(subject).to have_received(:run).with(:stop, '--name', name)
     end
   end
 
-    context 'lxc-attach is not supported' do
-      before do
-        subject.stub(attach: false, supports_attach?: false)
-        subject.stop
-      end
-
-      it 'runs a /sbin/halt within the container' do
-        expect(subject).to_not have_received(:attach)
-      end
-
-      it 'issues a lxc-stop with provided container name' do
-        expect(subject).to have_received(:run).with(:stop, '--name', name)
-      end
-    end
-  end
-
   describe 'state' do
     let(:name) { 'a-container' }
     subject    { described_class.new(sudo_wrapper, name) }
@@ -231,33 +206,4 @@ describe Vagrant::LXC::Driver::CLI do
 
     skip 'waits for the expected container state'
   end
-
-  describe 'check for whether lxc-attach is supported' do
-    let(:name) { 'a-running-container' }
-    subject    { described_class.new(sudo_wrapper, name) }
-
-    context 'lxc-attach is present on system' do
-      before { subject.stub(run: true) }
-
-      it 'returns true if `lxc-attach --name CNAME -- /bin/true` works' do
-        expect(subject.supports_attach?).to be_truthy
-        expect(subject).to have_received(:run).with(
-          :attach, '--name', name, '--', '/bin/true'
-        )
-      end
-    end
-
-    context 'lxc-attach is not present on system' do
-      before do
-        allow(subject).to receive(:run).and_raise(Vagrant::LXC::Errors::ExecuteError.new('msg'))
-      end
-
-      it 'returns true if `lxc-attach --name CNAME -- /bin/true` works' do
-        expect(subject.supports_attach?).to be_falsy
-        expect(subject).to have_received(:run).with(
-          :attach, '--name', name, '--', '/bin/true'
-        )
-      end
-    end
-  end
 end

spec/unit/driver_spec.rb

@@ -75,17 +75,6 @@ describe Vagrant::LXC::Driver do
     end
   end
 
-  describe 'supports_attach?' do
-    let(:cli) { double(Vagrant::LXC::Driver::CLI, supports_attach?: true) }
-
-    subject { described_class.new('name', nil, cli) }
-
-    it 'delegates to cli object' do
-      expect(subject.supports_attach?).to be_truthy
-      expect(cli).to have_received(:supports_attach?)
-    end
-  end
-
   describe 'start' do
     let(:customizations)         { [['a', '1'], ['b', '2']] }
     let(:internal_customization) { ['internal', 'customization'] }