prow/plugins/verify-owners: Default join-org URL to orgs/*/people
prow/plugins/verify-owners: Default join-org URL to orgs/*/people

Bringing over the current version of the defaulting that the
pull-request trigger has had since 2c80848bfd (Trust repo
collaborators unless disabled, 2018-06-30, #8527), to avoid an empty
link like:

  ...add them as <a href="">members</a> of...

when join_org_url was not configured.

michelle192837/test-infra,michelle192837/test-infra,BenTheElder/test-infra,michelle192837/test-infra,jessfraz/test-infra,jessfraz/test-infra,BenTheElder/test-infra,jessfraz/test-infra,cjwagner/test-infra,michelle192837/test-infra,fejta/test-infra,fejta/test-infra,cjwagner/test-infra,dims/test-infra,cblecker/test-infra,cjwagner/test-infra,fejta/test-infra,cblecker/test-infra,fejta/test-infra,BenTheElder/test-infra,BenTheElder/test-infra,kubernetes/test-infra,fejta/test-infra,kubernetes/test-infra,cjwagner/test-infra,BenTheElder/test-infra,cjwagner/test-infra,dims/test-infra,dims/test-infra,kubernetes/test-infra,cblecker/test-infra,cblecker/test-infra,cblecker/test-infra,dims/test-infra,kubernetes/test-infra,jessfraz/test-infra,BenTheElder/test-infra,fejta/test-infra,cjwagner/test-infra,kubernetes/test-infra,michelle192837/test-infra,jessfraz/test-infra,dims/test-infra,cblecker/test-infra,michelle192837/test-infra,kubernetes/test-infra,jessfraz/test-infra,dims/test-infra