Add a virtualenv_python argument to the pip module

This allows specifying the Python version to use when creating the
virtualenv. See issue #586.

packaging/language/pip.py

@@ -70,6 +70,14 @@ options:
         C(virtualenv2), C(~/bin/virtualenv), C(/usr/local/bin/virtualenv).
     required: false
     default: virtualenv
+  virtualenv_python:
+    version_added: "FIXME"
+    description:
+      - The Python executable used for creating the virtual environment.
+        For example C(python3.4), C(python2.7). When not specified, the
+        system Python version is used.
+    required: false
+    default: null
   state:
     description:
       - The state of module
@@ -224,6 +232,7 @@ def main():
             virtualenv=dict(default=None, required=False),
             virtualenv_site_packages=dict(default='no', type='bool'),
             virtualenv_command=dict(default='virtualenv', required=False),
+            virtualenv_python=dict(default=None, required=False, type='str'),
             use_mirrors=dict(default='yes', type='bool'),
             extra_args=dict(default=None, required=False),
             chdir=dict(default=None, required=False),
@@ -239,6 +248,7 @@ def main():
     version = module.params['version']
     requirements = module.params['requirements']
     extra_args = module.params['extra_args']
+    virtualenv_python = module.params['virtualenv_python']
     chdir = module.params['chdir']
 
     if state == 'latest' and version is not None:
@@ -256,18 +266,21 @@ def main():
             if module.check_mode:
                 module.exit_json(changed=True)
 
-            virtualenv = os.path.expanduser(virtualenv_command)
+            cmd = os.path.expanduser(virtualenv_command)
-            if os.path.basename(virtualenv) == virtualenv:
+            if os.path.basename(cmd) == cmd:
-                virtualenv = module.get_bin_path(virtualenv_command, True)
+                cmd = module.get_bin_path(virtualenv_command, True)
 
             if module.params['virtualenv_site_packages']:
-                cmd = '%s --system-site-packages %s' % (virtualenv, env)
+                cmd += ' --system-site-packages'
             else:
-                cmd_opts = _get_cmd_options(module, virtualenv)
+                cmd_opts = _get_cmd_options(module, cmd)
                 if '--no-site-packages' in cmd_opts:
-                    cmd = '%s --no-site-packages %s' % (virtualenv, env)
+                    cmd += ' --no-site-packages'
-                else:
+
-                    cmd = '%s %s' % (virtualenv, env)
+            if virtualenv_python:
+                cmd += ' -p%s' % virtualenv_python
+
+            cmd = "%s %s" % (cmd, env)
             this_dir = tempfile.gettempdir()
             if chdir:
                 this_dir = os.path.join(this_dir, chdir)