Placement microversion 1.35 gives us the root_required queryparam to GET
/allocation_candidates, allowing us to filter out candidates where the
*root* provider has/lacks certain traits, independent of traits
specified in any of the individual request groups.
Use it.
And add affordance for specifying such traits to the RequestSpec.
Which allows us to fix up the couple of request filters that were
hacking traits into the RequestSpec.flavor.
Change-Id: I44f02044ce178e84c23d178e5a23a3aa1208e502
bcc893a2b0