Strip extra '-' from md5sum output when creating S3 bucket

md5sum prints out the hash, followed by the filename. When piped in from
stdin, this equates to a '-' character.

cluster/aws/util.sh was incorrect including this '-' character as part
of the S3 bucket name, causing the script to fail on Linux machines with
the md5sum binary.

i.e. "s3://kubernetes-staging-0ac68d8c77915cc1069a9e2f5e1f1d2d -"

Fixed by using `awk` to return only the first column (up to the space)
Strip extra '-' from md5sum output when creating GCE projects
Merge pull request #2321 from jimmydivvy/fix_linux_md5sum

Strip extra '-' from md5sum output when creating S3 bucket