teuthology-suite: fix handling of extra YAML files

Previously these had to be absolute paths, or relative
paths had to be present in the pwd and in ceph-qa-suite.

With this change you can now pass a relative path
like 'debug/mds_client.yaml' to reference a debug
fragment from ceph-qa-suite.

Signed-off-by: John Spray <john.spray@redhat.com>