install: make overrides grouped by project

This lets us set different overrides for e.g. ceph vs samba, and makes it
so the schedule_teuthology.sh overrides don't specify a ceph sha1 for
samba installs.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit d7ec0892808848a4c1875499c1cfeb3f1dae701d)