Add hash_salt_path parameter

`hash_salt_path` parameter is for security.

Here is a use cases.

Use case: Don't version control hash salt.

If we version control fluentd.conf, hash salt is also version controlled
because we need to write hash salt by `hash_salt` parameter in
fluentd.conf.

We can use random hash salt for each host by `hash_salt_path` parameter
without version controlling hash salt. Because `hash_salt_path` generates
random hash salt key and saves it when the path doesn't exist.