ansible_local: Polish the new `pip_args` option

Ref GH-8170

plugins/provisioners/ansible/cap/guest/pip/pip.rb

@@ -6,14 +6,16 @@ module VagrantPlugins
         module Pip
 
           def self.pip_install(machine, package, version = "", pip_args = "", upgrade = true)
-            upgrade_arg = "--upgrade " if upgrade
+            upgrade_arg = "--upgrade" if upgrade
             version_arg = ""
 
             if !version.to_s.empty? && version.to_s.to_sym != :latest
               version_arg = "==#{version}"
             end
 
-            machine.communicate.sudo "pip install #{pip_args} #{upgrade_arg}#{package}#{version_arg}"
+            args_array = [pip_args, upgrade_arg, "#{package}#{version_arg}"]
+
+            machine.communicate.sudo "pip install #{args_array.join(' ')}"
           end
 
           def self.get_pip(machine)

website/source/docs/provisioning/ansible_local.html.md

@@ -79,8 +79,9 @@ This section lists the _specific_ options for the Ansible Local provisioner. In
 
     The default value is `:default`, and any invalid value for this option will silently fall back to the default value.
 
-- `pip_args` (string) - When Ansible is installed via pip this option allows the defition of additional pip arguments to be passed along on the
-command line (for example, [--index-url](https://pip.pypa.io/en/stable/reference/pip_install/#cmdoption-i)).
+- `pip_args` (string) - When Ansible is installed via pip, this option allows the definition of additional pip arguments to be passed along on the command line (for example, [`--index-url`](https://pip.pypa.io/en/stable/reference/pip_install/#cmdoption-i)).
+
+    By default, this option is not set.
 
 - `provisioning_path` (string) - An absolute path on the guest machine where the Ansible files are stored. The `ansible-galaxy` and `ansible-playbook` commands are executed from this directory. This is the location to place an [ansible.cfg](http://docs.ansible.com/ansible/intro_configuration.html) file, in case you need it.