Adds ApplicationBuilder.Builder.appImpl
Adds MovableElasticWebAppCluster.TIME_TO_LIVE_SECONDS
Supports customising installer downloads

- Adds DownloadsRegistry (a set of "resolvers" for figuring out
  the download URL, given an EntityDriver)
- Adds Attributes.DOWNLOAD_URL for the default download pattern
  of a given entity (where ${version} etc will get substituted)
Customise installer downloads through brooklyn.properties
Use DownloadsRegistry in entity drivers

- Ands defines default DOWNLOAD_URL on each entity