lib/3.7/bundles.cf: add maybe_run convenience bundle
Merge pull request #356 from tzz/tzz/maybe_run

lib/3.7/bundles.cf: add maybe_run convenience bundle
Add: backport conveniance bundle to run a bundle if its defined

Sometimes you don't know if a bundle is defined in your policy. Maybe
because you are using dynamic inputs. This bundle makes it easy to
activate a bundle if it happens to be defined.
Merge pull request #357 from nickanderson/run_ifdefined

Add: conveniance bundle to run a bundle if its defined