MODE-2368

* Extends vdb sequencer functionality to support the processing of
  dynamic vdbs.

* The syntax of a dynamic vdb is identical to that of a container vdb's
  manifest hence the extension of the VdbSequencer by the
  VdbDynamicSequencer.

* Adds a twitter test that takes the declarative model as a dynamic vdb
  and sequences it accordingly.
Please watch the spacing, and be sure to never use tabs.