Pass extra vars via file rather than via commandline, including custom creds.

The extra vars file created lives in the playbook private runtime
directory, and will be reaped along with the rest of the directory.

Adjust assorted unit tests as necessary.

awx/main/utils/common.py

@@ -803,6 +803,8 @@ def wrap_args_with_proot(args, cwd, **kwargs):
         if not os.path.exists(path):
             continue
         path = os.path.realpath(path)
+        if os.path.isdir(path):
+            path = os.path.join(path, '')  # add a trailing slash
         new_args.extend(['--bind', '%s' % (path,), '%s' % (path,)])
     if kwargs.get('isolated'):
         if 'ansible-playbook' in args: