Fix lxc-attach --namespaces support detection and cache the result

lib/vagrant-lxc/driver/cli.rb

@@ -82,12 +82,14 @@ module Vagrant
             opts       = cmd.pop
             namespaces = Array(opts[:namespaces]).map(&:upcase).join('|')
 
-            if run(:attach, '-h').include?('--namespaces')
+            if namespaces
+              if supports_attach_with_namespaces?
                 extra = ['--namespaces', namespaces] if namespaces
               else
                 raise LXC::Errors::NamespacesNotSupported
               end
             end
+          end
 
           run :attach, '--name', @name, *((extra || []) + cmd)
         end
@@ -114,6 +116,14 @@ module Vagrant
         def run(command, *args)
           @sudo_wrapper.run("lxc-#{command}", *args)
         end
+
+        def supports_attach_with_namespaces?
+          unless defined?(@supports_attach_with_namespaces)
+            @supports_attach_with_namespaces = run(:attach, '-h', '2>&1').include?('--namespaces')
+          end
+
+          return @supports_attach_with_namespaces
+        end
       end
     end
   end

spec/unit/driver/cli_spec.rb

@@ -146,13 +146,13 @@ describe Vagrant::LXC::Driver::CLI do
     end
 
     it 'supports a "namespaces" parameter' do
-      subject.stub(:run).with(:attach, '-h').and_return('--namespaces')
+      subject.stub(:run).with(:attach, '-h', '2>&1').and_return('--namespaces')
       subject.attach *(command + [{namespaces: ['network', 'mount']}])
       subject.should have_received(:run).with(:attach, '--name', name, '--namespaces', 'NETWORK|MOUNT', '--', *command)
     end
 
     it 'raises a NamespacesNotSupported error if not supported' do
-      subject.stub(:run).with(:attach, '-h').and_return('not supported')
+      subject.stub(:run).with(:attach, '-h', '2>&1').and_return('not supported')
       expect {
         subject.attach *(command + [{namespaces: ['network', 'mount']}])
       }.to raise_error(Vagrant::LXC::Errors::NamespacesNotSupported)