MODE-2394: Updates to the Vdb Sequencer in line with Teiid Schema

* Updates the Vdb sequencer's cnd model and lexicon to reflect the schema
  included in the latest version of teiid.
The purpose of the change is to allow multiple vdb:source objects in a vdb:virtualDatabase. The basis for this change is Teiid's [vdb-deployer.xsd](https://github.com/teiid/teiid/blob/master/client/src/main/resources/vdb-deployer.xsd) schema, which defines the specification of the vdb xml. Looking at the history of this file, the source has been 0-unbounded since it was migrated to github (March 2010). Thus, in retrospect I believe this to be a small oversight when the cnd / sequencer was originally developed.
Certainly, the fix can include deprecating the original properties but that would imply any new code would have to check for population of these deprecated properties as well as existing code checking for population of the sources node. Would it be simpler and cleaner to highlight this breaking change in the usual forms, eg. documentation, and maybe even provide a migration tool to 'fix' existing implementations?