Make WORKSPACE_BASE_DIR configurable
Make WORKSPACE_BASE_DIR configurable

For example, when storing the dependent releases in a different path.

Signed-off-by: Gerhard Lazu <1dbacecb153173531fe28399d2eb60d6542a913b@pivotal.io>

pivotal-cf/cf-rabbitmq-release,pivotal-cf/cf-rabbitmq-release,pivotal-cf/cf-rabbitmq-release,pivotal-cf/cf-rabbitmq-release