Heat does not support Keystone Trusts when deployed in standalone
mode. This change forces an error when HEAT_DEFERRED_AUTH is set
to anything other than "password" if HEAT_STANDALONE is True and
advises of the acceptable setting.
Change-Id: Ib4ee9d9af396093137a2a0f99f1b18ae153ccdb3
Closes-Bug: #1463837