Make Situ ES instance optional

[mikeiannacone]

Right now, the SITU_DB_CONFIG env var (and a valid Elasticsearch instance) is needed, even if no situ instance is present.

It should be a relatively easy fix to make this optional & configurable, instead of assuming it is present. The check here will need to be changed, and the querySitu , getVertexSitu , getInEdgesFromSitu, and getOutEdgesFromSitu methods in the DBConnectionResource.java will need to be modified to return empty results when not connected.