Use stager_minimum_task_memory from config rather than hardcoding 1024

The previous logic hard-coded 1024, which works when dea.yml/staging:memory_limit_mb isn't overridden, because that's the [default value](https://github.com/cloudfoundry/dea_ng/blob/dca6f83aab94d5b780b68e742f4eb5e98681a000/lib/dea/config.rb#L158) of the config, but is wrong if the property was [overridden](https://github.com/cloudfoundry/cf-release/blob/master/jobs/dea_next/templates/dea.yml.erb#L65).