core: Vagrant.has_plugin? to check if plugin installed [GH-1736]

CHANGELOG.md

@@ -35,6 +35,8 @@ FEATURES:
   - NFS: Arbitrary export options can be specified using
    `bsd__nfs_options` and `linux__nfs_options`. [GH-1029]
   - Static IP can now be set on public networks. [GH-1745]
+  - Add `Vagrant.has_plugin?` method for use in Vagrantfile to check
+    if a plugin is installed. [GH-1736]
 
 IMPROVEMENTS:
 

lib/vagrant.rb

@@ -132,6 +132,13 @@ module Vagrant
     Config.run(version, &block)
   end
 
+  # This checks if a plugin with the given name is installed. This can
+  # be used from the Vagrantfile to easily branch based on plugin
+  # availability.
+  def self.has_plugin?(name)
+    plugin("2").registered.include?(name)
+  end
+
   # Returns a superclass to use when creating a plugin for Vagrant.
   # Given a specific version, this returns a proper superclass to use
   # to register plugins for that version.