Allow using custom requirements trees.
Allow using custom requirements trees.

This makes testing with unlanded requirements patches way easier.

Change-Id: Ia3b986a9eb58d93ad239ef077a6069be958170a0

noorul/os-project-config,openstack-infra/project-config,coolsvap/project-config,coolsvap/project-config,Tesora/tesora-project-config,noorul/os-project-config,Tesora/tesora-project-config,anbangr/osci-project-config,anbangr/osci-project-config,openstack-infra/project-config,dongwenjuan/project-config,dongwenjuan/project-config