Make S3 signed URL expiry configurable, with 60s default

Per JENKINS-23897, S3 signed URLs for downloads were very sensitive to server
clock drift / time sync issues. The hardcoded 4 second expiry was too short even
if the server time was accurate if Jenkins took a moment to respond and/or
the request to S3 took a moment. On high latency links like 3G downloads frequently
failed.

Make the timeout configurable under Advanced, and default to 60 seconds instead
of 4.
Merge pull request #33 from ringerc/longer-url-expiry

Make S3 signed URL expiry configurable, with 60s default