core: don't allow '/' in VM name [GH-4289]

plugins/kernel_v2/config/vm.rb

@@ -662,7 +662,7 @@ module VagrantPlugins
 
         # Validate sub-VMs if there are any
         @__defined_vms.each do |name, _|
-          if name =~ /[\[\]\{\}]/
+          if name =~ /[\[\]\{\}\/]/
             errors["vm"] << I18n.t(
               "vagrant.config.vm.name_invalid",
               name: name)

test/unit/plugins/kernel_v2/config/vm_test.rb

@@ -132,6 +132,7 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
     [
       "foo [1]",
       "bar {2}",
+      "foo/bar",
     ].each do |name|
       it "should disallow names with brackets" do
         subject.define name