Based on the contents of brooklyn properties, sets up rules for resolving where to
 download artifacts from, for installing entities. 
 
 By default, these rules override the DOWNLOAD_URL defined on the entities in code.
 Global properties can be specified that apply to all entities. Entity-specific properties
 can also be specified (which override the global properties for that entity type).
 
 Below is an example of realistic configuration for an enterprise who have an in-house 
 repository that must be used for everything, rather than going out to the public internet. 
 
 // FIXME Check format for including addonname- only if addonname is non-null?
 // FIXME Use this in a testng test case
 brooklyn.downloads.all.url=http://downloads.acme.com/brookyn/repository/${simpletype}/${simpletype}-${addon?? addon-}${version}.${fileSuffix!.tar.gz}
 
 
 
 To illustrate the features and variations one can use, below is an example of global 
 properties that can be specified. The semicolon-separated list of URLs will be tried  in-order
 until one succeeds. The fallback url says to use that if all other URLs fail (or no others are
 specified). 
 
 brooklyn.downloads.all.url=http://myurl1/${simpletype}-${version}.tar.gz; http://myurl2/${simpletype}-${version}.tar.gz
 brooklyn.downloads.all.fallbackurl=http://myurl3/${simpletype}-${version}.tar.gz
 
 
 
 Similarly, entity-specific properties can be defined. All "global properties" will also apply
 to this entity type, unless explicitly overridden.
 
 brooklyn.downloads.entity.tomcatserver.url=http://mytomcaturl1/tomcat-${version}.tar.gz
 brooklyn.downloads.entity.tomcatserver.fallbackurl=http://myurl2/tomcat-${version}.tar.gz
 
 
 
 Downloads for entity-specific add-ons can also be defined. All "global properties" will also apply
 to this entity type, unless explicitly overridden.
 
 brooklyn.downloads.entity.nginxcontroller.addon.stickymodule.url=http://myurl1/nginx-stickymodule-${version}.tar.gz
 brooklyn.downloads.entity.nginxcontroller.addon.stickymodule.fallbackurl=http://myurl2/nginx-stickymodule-${version}.tar.gz
 
 
 
 If no explicit URLs are supplied, then by default it will use the DOWNLOAD_URL attribute
 of the entity  (if supplied), followed by the fallbackurl if that fails. 
 
 A URL can be a "template", where things of the form ${version} will be substituted for the value
 of "version" provided for that entity. The freemarker template engine is used to convert URLs 
 (see 
Entity instance
   
driver: the EntityDriver instance being used for the Entity
   simpletype: the unqualified name of the entity type
   type: the fully qualified name of the entity type
   addon: the name of the entity add-on, or null if it's the core entity artifact
   version: the version number of the entity to be installed (or of the add-on)