sahara: explicitly set the list of repos in PROJECTS
sahara: explicitly set the list of repos in PROJECTS

Set the list of projects inside $PROJECTS (when not already available)
instad of relying on DEVSTACK_GATE_SAHARA, which will go away at some
point in the future.

Change-Id: I65897fa2c562645e4a0ce3cac03ff7a2d475308f

openstack-infra/project-config,openstack-infra/project-config