provide an option to override sbt.ivy.home

Problem: 
Want to override the sbt.ivy.home so that I can use same ivy home for my Play and SBT projects.  But once you hardcode the ivy-home in the sbt.boot.properties file you cannot override it any more. SBT will ignore -Dsbt.ivy.home property you pass from command line

One way to tackle this problem is to use `${sbt.ivy.home-${play.home}/../repository}`. In this case SBT will try to read the sbt.ivy.home property and if not specified use the ${play.home}../repository. This will allow developers to configure the sbt.ivy.home and default to play.home repository.