add a configuration option to skip creating source vertices present only in edge input

In giraph source vertices present only in edge input are created by default.
In this patch an additional configuration parameter has been added to control this behavior. The default behavior is still to create the source vertex if not already present, but the user can choose not to do so, by setting CREATE_EDGE_SOURCE_VERTICES BoolConfoption to false.

An example use case for such a change is to help in creating subgraphs of the original graph based on a sampled vertex input.