Added configuration options which limited of the app instances spawned per dea.

This is an integral part of the change set which aims at giving users the a way to limit the number of app instances spawned per DEA.  This pull request tried to expose the necessary configuration for the feature.

Other pull requests:
cloudfoundry / cloud_controller_ng:   https://github.com/cloudfoundry/cloud_controller_ng/pull/134
cloudfoundry / dea_ng:                      https://github.com/cloudfoundry/dea_ng/pull/85