Add --no-delete to 'snapshot pop' command

In our test environments, it's good to be able to roll back to the same,
anonymous, snapshot repeatedly.  This patch adds a `--no-delete` option
to the `snapshot pop` command allowing this.

This makes the new core snapshot behaviour more consistent with what we
were doing with vagrant-multiprovider-snap
(https://github.com/scalefactory/vagrant-multiprovider-snap)

plugins/commands/snapshot/command/pop.rb

@@ -10,17 +10,22 @@ module VagrantPlugins
         include PushShared
 
         def execute
+          options = {}
           opts = OptionParser.new do |o|
             o.banner = "Usage: vagrant snapshot pop [options] [vm-name]"
             o.separator ""
             o.separator "Restore state that was pushed with `vagrant snapshot push`."
+
+            o.on("--no-delete", "Don't delete the snapshot after the restore") do
+                options[:no_delete] = true
+            end
           end
 
           # Parse the options
           argv = parse_options(opts)
           return if !argv
 
-          return shared_exec(argv, method(:pop))
+          return shared_exec(argv, method(:pop), options)
         end
       end
     end

plugins/commands/snapshot/command/push_shared.rb

@@ -4,7 +4,7 @@ module VagrantPlugins
   module CommandSnapshot
     module Command
       module PushShared
-        def shared_exec(argv, m)
+        def shared_exec(argv, m, opts={})
           with_target_vms(argv) do |vm|
             if !vm.id
               vm.ui.info("Not created. Cannot push snapshot state.")
@@ -12,7 +12,7 @@ module VagrantPlugins
             end
 
             vm.env.lock("machine-snapshot-stack") do
-              m.call(vm)
+              m.call(vm,opts)
             end
           end
 
@@ -20,14 +20,14 @@ module VagrantPlugins
           0
         end
 
-        def push(machine)
+        def push(machine,opts={})
           snapshot_name = "push_#{Time.now.to_i}_#{rand(10000)}"
 
           # Save the snapshot. This will raise an exception if it fails.
           machine.action(:snapshot_save, snapshot_name: snapshot_name)
         end
 
-        def pop(machine)
+        def pop(machine,opts={})
           # By reverse sorting, we should be able to find the first
           # pushed snapshot.
           name = nil
@@ -45,11 +45,16 @@ module VagrantPlugins
             return
           end
 
-          # Restore the snapshot and tell the provider to delete it as well.
+          snapshot_delete = true
+          if opts.key?(:no_delete)
+              snapshot_delete = false
+          end
+
+          # Restore the snapshot and tell the provider to delete it, if required
           machine.action(
             :snapshot_restore,
             snapshot_name: name,
-            snapshot_delete: true)
+            snapshot_delete: snapshot_delete)
         end
       end
     end