Remove head slash of S3 path configure

If we put 'path' starts with '/', it uploads a file to empty name
directory. For example, we set path as '/path/log', S3 path will be
'//path/log/...'.

This commit wiil remove head slash if given.
Merge branch 's3_path' of git://github.com/adorechic/fluent-plugin-redshift into adorechic-s3_path