Update DCOS_SPACE document (#1311)
Update DCOS_SPACE document (#1311)

* Update DCOS_SPACE document

* Add a table about DCOS_SPACE similar to https://docs.mesosphere.com/1.9/security/#secrets

* Change "application" to "service" in the doc

* little edits

vishnu2kmohan/dcos-commons,vishnu2kmohan/dcos-commons,vishnu2kmohan/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,vishnu2kmohan/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,vishnu2kmohan/dcos-commons