[JENKINS-19587] Support uploading mulitple file entries

Use standard Jenkins form display and processing facilities to get dynamic select box population and reduce overall code.
@dmbeer If I use Regions#getName, uploads fail with the following error:

java.lang.IllegalArgumentException: No enum constant com.amazonaws.regions.Regions.us-west-1

Whereas the code as written works:

Publish artifacts to S3 Bucket bucket=cbdevtest, file=foo region=US_EAST_1, upload from slave=false managed=false