Explain labels in User Guide
Magnum labels has many possible key/value that have specific meaning.
It is helpful to summarize them in a table for quick reference.
Also add details on labels specific to Mesos that have not been
documented before.
Partially implements: blueprint user-guide
Co-Authored-By: Ton Ngo <ton@us.ibm.com>
Change-Id: If727563d3cb93663b696abb707cb2acb76a693e2