Use trellis-cli for Vagrant galaxy install

This replaces the default galaxy command (`ansible-galaxy install`) with
the much better `trellis galaxy install` if trellis-cli is available on
the host machine.

With trellis-cli's command, there should be no reason to use the
annoying `SKIP_GALAXY` env var which was added to prevent Vagrant from
always installing the roles each time. `trellis galaxy install` is
efficient and quickly skips already installed roles.

Vagrantfile

@@ -131,6 +131,10 @@ Vagrant.configure('2') do |config|
     ansible.galaxy_role_file = File.join(provisioning_path, 'galaxy.yml') unless vconfig.fetch('vagrant_skip_galaxy') || ENV['SKIP_GALAXY']
     ansible.galaxy_roles_path = File.join(provisioning_path, 'vendor/roles')
 
+    if which('trellis')
+      ansible.galaxy_command = 'trellis galaxy install'
+    end
+
     ansible.groups = {
       'web' => ['default'],
       'development' => ['default']