Expose umask override value as config parameter: localstorage.umask

Expose umask override value as config parameter: localstorage.umask

Commit 451c06d introduced override for umask value.
This is needed to avoid broken env configuration or dirty workers
to mess with the permissions when creating new files.

Most Nextcloud, that does not integrate with external software
would work fine with an hard-coded value (451c06d set it at 022).

Advanced install may require more flexibility, as such this commit
exposes the "umask override value" as configuration parameter:
`localstorage.umask`

It defaults to 0022 both in code and in config/config.sample.php .

Signed-off-by: Martin Brugnara <martin@0x6d62.eu>