For kubernetes/test-infra#476.

Also, put the repo in the build metadata so Gubernator can link to the
right thing for postsubmit jobs.

For PR #$NUM in a given repo, the logs will go to these places:

	kubernetes/kubernetes => /pr-logs/pull/$NUM
	kubernetes/test-infra => /pr-logs/pull/test-infra/$NUM
	google/cadvisor => /pr-logs/pull/google_cadvisor/$NUM

Tested with a manual harness. Note that this leaves the latest/indirect
untouched as well as postsubmit jobs-- *job names* are still unique, the
main issue was that PR numbers might collide. Disturbing as little
structure as possible reduces how many things will break.