Enhance viz-workflow for serial and parallel runs

[rushirajnenuji]

There are some discrepancies in the output artifacts generated by viz-workflow.

Applies to parallel interface added by #165:

• Make sure tile discovery steps are only executed if specific workflow flags are enabled.
• Generate required metadata files (based on config):
  • Cesium top level tileset.json for 3dtiles
  • WMTS Capabilities doc post all viz-workflow steps are executed.