Single batch run option: add an interval override for bucket listing.

[pranspach]

If watch_for_new_files is set to false, the plugin will only list the s3 bucket once and exit after processing completes.

This is consistent with the file Logstash plugin which exits after successfully processing files.

I opened a ticket for discussion. Please review -- I'd like to work with you guys to add this functionality. @yaauie

• [x] Signed CLA
• [x] Tested under single file and multiple file conditions
• [x] Verified that s3 list only happens once

[pranspach]

Thanks @yaauie -- fixed.

[pranspach]

@yaauie Any more feedback on this PR? Your suggested change that I implemented has been working well for me.

[yaauie]

@pranspach I've pulled this into a new PR (#162) that adds docs etc., and will merge it once the build goes green.

Thanks for your contribution :tada: