Adding new options for eb.yml that were recently added to eb_deployer

Adding the following new options which were added to the eb_deployer. These options are all optional and can be excluded from the eb.yml

package_bucket - Name of s3 bucket where uploaded application packages will be stored. Note that the string ".packages" will be added as a suffix to your bucket. So, if "thoughtworks.simple" is passed as the bucket name, the actual s3 bucket name will be thoughtworks.simple.packages.

version_prefix - Specifies a prefix to prepend to the version label. This can be useful if you want to use different binaries for different environments.

keep_latest - Specifies the maximum number of versions to keep.  Older versions are removed and deleted from the S3 source bucket as well. If specified as zero or not specified, all versions will be kept.  If a version_prefix is given, only removes version starting with the prefix.