Vagrantfile: specify full path to using_sshfs

When running `vagrant ssh-config` from ansible, Vagrant for some reason
still wants to execute the File.write() operation, but fails to do so
because the $PWD is $topsrcdir/vagrant, in which .vagrant does not
exist.

Switch to using the absolute path.

Closes: #555
Approved by: cgwalters

Vagrantfile

@@ -14,7 +14,7 @@ Vagrant.configure(2) do |config|
 
     if Vagrant.has_plugin?('vagrant-sshfs')
       config.vm.synced_folder ".", "/var/roothome/sync", type: 'sshfs'
-      File.write('.vagrant/using_sshfs', '')
+      File.write(__dir__ + '/.vagrant/using_sshfs', '')
     end
 
     # turn off the default rsync in the vagrant box

vagrant/setup.yml

@@ -4,7 +4,7 @@
   become: yes
   tasks:
     - name: generate config
-      local_action: shell vagrant ssh-config > ssh-config
+      local_action: shell vagrant ssh-config > ../ssh-config
       become: no
 
     # The test suite requires direct ssh to root.