vagrant: obey VAGRANT_CWD when completing vagrant commands (#7219)

plugins/vagrant/_vagrant

@@ -69,8 +69,8 @@ __box_list ()
 
 __vm_list ()
 {
-    _wanted application expl 'command' compadd $(command grep Vagrantfile -oe '^[^#]*\.vm\.define *[:"]\([a-zA-Z0-9_-]\+\)' 2>/dev/null | awk '{print substr($2, 2)}')
-    _wanted application expl 'command' compadd $(command ls .vagrant/machines/ 2>/dev/null)
+    _wanted application expl 'command' compadd $(command grep "${VAGRANT_CWD:-.}/Vagrantfile" -oe '^[^#]*\.vm\.define *[:"]\([a-zA-Z0-9_-]\+\)' 2>/dev/null | awk '{print substr($2, 2)}')
+    _wanted application expl 'command' compadd $(command ls "${VAGRANT_CWD:-.}/.vagrant/machines/" 2>/dev/null)
 }
 
 __vagrant-box ()