suite: return [] for build_matrix base case

This gives us an empty list instead of a crash on an empty dir
or bad symlink

Signed-off-by: Sage Weil <sage@inktank.com>
schedule_suite.sh: allow partial 'suite' names

Mangle the name (/ -> :) in the run name.  Separate base from
relative path so that the job names don't include a full path.

Signed-off-by: Sage Weil <sage@inktank.com>