suite: fix build_matrix when non-yaml present in % dir

First, if we get a non-yaml file, return an empty list.

Second, if we are in a % mode, exclude an empty raw result from the
combination.  Otherwise, we get no resulting jobs.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #304 from ceph/wip-fix-build-matrix

suite: fix build_matrix when non-yaml present in % dir