Sticky buildmode implementation can conflict with use of Sticky as a state signifier

The Sticky property on a node usually only indicates that the node should be pinned to the top of lists. The actual buildmode that a node has in that context could be one of many depending on other factors. The presence of sticky buildmode can break this paradigm, and the selection of that buildmode is hardcoded into _nd_preprocess_node.

The fix for this issue is has two parts:

1. Allow developers to ignore sticky display selection using a variable (this issue)
2. Change the ds implementation to allow buildmode selection based on node state (#1316608: Allow buildmode switching based on arbitrary node state (e.g. Sticky, Promoted, Unpublished)