suite: do not calculate product on an empty list

This fixes a bug where a directory with just % generates a
bogus job item with no actual content.  (e.g.,

$ find basic
basic
basic/%
$ teuthology-suite --dry-run --base . --collections basic --name foo
INFO:teuthology.suite:Collection basic in ./basic
INFO:teuthology.suite:configs [('basic/{}', [])]

...which then blows up because the job yaml is empty.  With
the fix, there are no generated jobs:

$ teuthology-suite --dry-run --base . --collections basic --name foo
INFO:teuthology.suite:Collection basic in ./basic
INFO:teuthology.suite:configs []

(The configs print was temporarily added for debugging
purposes.)

Signed-off-by: Sage Weil <sage@inktank.com>
suite: be verbose about how many jobs we generate

Signed-off-by: Sage Weil <sage@inktank.com>
suite: don't schedule follow-on summary job for an empty run

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #91 from ceph/wip-suite

suite: do not calculate product on an empty list