Hook validation: skip leading spaces/newlines

Improves the situation with #3020 a bit.

Does nothing about other valid shell commands that the current
validation would reject:

  - shell builtins like --post-hook 'if [ -x /my/script ]; then /my/script; fi'
  - variable assignments like --post-hook 'ENV_VAR=value command'
  - comments
  - redirections like --post-hook '<infile command'
  - I'm sure I'm missing many other exciting possibilities

certbot/hooks.py

@@ -27,7 +27,7 @@ def _validate_hook(shell_cmd, hook_name):
     :raises .errors.HookCommandNotFound: if the command is not found
     """
     if shell_cmd:
-        cmd = shell_cmd.partition(" ")[0]
+        cmd = shell_cmd.split(None, 1)[0]
         if not _prog(cmd):
             path = os.environ["PATH"]
             msg = "Unable to find {2}-hook command {0} in the PATH.\n(PATH is {1})".format(