Add ansible_local support for non-Windows

Previously only Windows would use Vagrant's `ansible_local` provisioner.
But it's a much easier experience to get started in development since it
skips the Ansible requirement.

This automatically uses the `ansible_local` provisioner if
`ansible-playbook` does not exist on the host machine.

CHANGELOG.md

@@ -1,4 +1,5 @@
 ### HEAD
+* Add ansible_local support for non-Windows ([#824](https://github.com/roots/trellis/pull/824))
 * Option to install WP-CLI packages ([#837](https://github.com/roots/trellis/pull/837))
 * Update WP-CLI to 1.2.1 ([#838](https://github.com/roots/trellis/pull/838))
 * Auto-install Vagrant plugins ([#829](https://github.com/roots/trellis/pull/829))

Vagrantfile

@@ -91,11 +91,11 @@ Vagrant.configure('2') do |config|
     end
   end
 
-  provisioner = Vagrant::Util::Platform.windows? ? :ansible_local : :ansible
-  provisioning_path = Vagrant::Util::Platform.windows? ? ANSIBLE_PATH_ON_VM : ANSIBLE_PATH
+  provisioner = local_provisioning? ? :ansible_local : :ansible
+  provisioning_path = local_provisioning? ? ANSIBLE_PATH_ON_VM : ANSIBLE_PATH
 
   config.vm.provision provisioner do |ansible|
-    if Vagrant::Util::Platform.windows?
+    if local_provisioning?
       ansible.install_mode = 'pip'
       ansible.provisioning_path = provisioning_path
       ansible.version = vconfig.fetch('vagrant_ansible_version')

lib/trellis/vagrant.rb

@@ -62,6 +62,10 @@ def load_wordpress_sites
   wordpress_sites
 end
 
+def local_provisioning?
+  @local_provisioning ||= Vagrant::Util::Platform.windows? || !which('ansible-playbook') || ENV['FORCE_ANSIBLE_LOCAL']
+end
+
 def local_site_path(site)
   File.expand_path(site['local_path'], ANSIBLE_PATH)
 end
@@ -90,3 +94,16 @@ def post_up_message
 def remote_site_path(site_name, site)
   "/srv/www/#{site_name}/#{site['current_path'] || 'current'}"
 end
+
+def which(cmd)
+  exts = ENV['PATHEXT'] ? ENV['PATHEXT'].split(';') : ['']
+
+  paths = ENV['PATH'].split(File::PATH_SEPARATOR).flat_map do |path|
+    exts.map { |ext| File.join(path, "#{cmd}#{ext}") }
+  end
+
+  paths.any? do |path|
+    next unless File.executable?(path) && !File.directory?(path)
+    system("#{path} --help", %i(out err) => File::NULL)
+  end
+end