Updates to bindep fallback for Xenial
Updates to bindep fallback for Xenial

We assumed a precise/trusty split in the bindep fallback file in many
places, but now we need to handle precise, trusty, and xenial. Correct
this by defaulting to using just trusty on ubuntu packages and
explicitly exclude precise where it is not needed or will not function.

Change-Id: Ib44fd82f282d8b9115fc25dcf77e6b0676a886a3

dongwenjuan/project-config,dongwenjuan/project-config,openstack-infra/project-config,Tesora/tesora-project-config,openstack-infra/project-config,Tesora/tesora-project-config