Add one level of indirection in the ecosystem so that we can use the project to drive ecosystem test builds without hitting the downloads.scala-ide.org website.

Right now the root URI for update sites (i.e. downloads.scala-ide.org) is hardcoded in the ecosystem.conf file the idea would be to have one level of indirection for that root URI that would be maven-overridable in a pom.xml option. For test builds, we would point to fresh builds by overriding that option with a local URI.