Change log level when chmod fails ?

Hi,

we have a kind of restricted nginx setup (non root both master and workers), so lua-resty-auto-ssl fails chmoding (though) the permissions are ok, and everything works):

`nginx: [error] [lua] init_master.lua:56: generate_config(): auto-ssl: failed to create letsencrypt dir permissions: Executing command failed: chmod 777 /etc/resty-auto-ssl/letsencrypt: chmod: /etc/resty-auto-ssl/letsencrypt: Operation not permitted`

IMHO this could be changed to warn only:

- 777 is a bit of overkill (but I understand the simplicity)
- If this failed, it does not mean that lets encrypt will actually fail

Would you accept a simple PR? Or you think that it's ok as it is?

Start at init_master.lua:56 in generate_config() and inspect how the failed chmod is logged. Reproduce the restricted nginx setup if possible, then confirm that a chmod failure is reported at warning level while the rest of configuration generation continues as described.

Written by the indexing model from the issue text.