Add a subscription mechanism for config updates
Add a subscription mechanism for config updates

A number of components could benefit from being able to monitor
configuration updates without implementing their own polling. This
change allows a consumer of the config agent to subscribe to changes
in the configuration and inspect the delta between configurations when
they change.

Signed-off-by: Steve Kuznetsov <a7da0fca91d9933d2725d731a48cabdc9e8ffd7a@redhat.com>

cjwagner/test-infra,kargakis/test-infra,jlowdermilk/test-infra,brahmaroutu/test-infra,ixdy/kubernetes-test-infra,kargakis/test-infra,cblecker/test-infra,jlowdermilk/test-infra,fejta/test-infra,lavalamp/test-infra,cblecker/test-infra,kubernetes/test-infra,fejta/test-infra,jlowdermilk/test-infra,jessfraz/test-infra,jessfraz/test-infra,kargakis/test-infra,michelle192837/test-infra,krzyzacy/test-infra,ixdy/kubernetes-test-infra,lavalamp/test-infra,shyamjvs/test-infra,cblecker/test-infra,fejta/test-infra,michelle192837/test-infra,kargakis/test-infra,lavalamp/test-infra,BenTheElder/test-infra,michelle192837/test-infra,lavalamp/test-infra,brahmaroutu/test-infra,dims/test-infra,cblecker/test-infra,brahmaroutu/test-infra,kubernetes/test-infra,BenTheElder/test-infra,shyamjvs/test-infra,monopole/test-infra,jessfraz/test-infra,shyamjvs/test-infra,cblecker/test-infra,krzyzacy/test-infra,jessfraz/test-infra,fejta/test-infra,ixdy/kubernetes-test-infra,fejta/test-infra,ixdy/kubernetes-test-infra,kargakis/test-infra,shyamjvs/test-infra,monopole/test-infra,monopole/test-infra,BenTheElder/test-infra,dims/test-infra,kubernetes/test-infra,cjwagner/test-infra,cjwagner/test-infra,ixdy/kubernetes-test-infra,dims/test-infra,kubernetes/test-infra,BenTheElder/test-infra,michelle192837/test-infra,kargakis/test-infra,jessfraz/test-infra,krzyzacy/test-infra,cjwagner/test-infra,jlowdermilk/test-infra,brahmaroutu/test-infra,dims/test-infra,pwittrock/test-infra,kubernetes/test-infra,shyamjvs/test-infra,cblecker/test-infra,krzyzacy/test-infra,krzyzacy/test-infra,cjwagner/test-infra,dims/test-infra,fejta/test-infra,pwittrock/test-infra,brahmaroutu/test-infra,kubernetes/test-infra,shyamjvs/test-infra,pwittrock/test-infra,lavalamp/test-infra,pwittrock/test-infra,monopole/test-infra,BenTheElder/test-infra,lavalamp/test-infra,jlowdermilk/test-infra,cjwagner/test-infra,BenTheElder/test-infra,michelle192837/test-infra,monopole/test-infra,dims/test-infra,jlowdermilk/test-infra,pwittrock/test-infra,michelle192837/test-infra,jessfraz/test-infra,krzyzacy/test-infra,monopole/test-infra,brahmaroutu/test-infra