x/build: tons of kubernetes replicasets

[bradfitz]

Our deployment tooling is leaking old replicasets:

bradfitz@gdev:~/go$ k get rs | wc -l
154
bradfitz@gdev:~/go$ k get rs | head -40
NAME                                 DESIRED   CURRENT   READY     AGE
coordinator-deployment-1259164551    0         0         0         1y
coordinator-deployment-1291916527    0         0         0         1y
coordinator-deployment-1389041072    0         0         0         356d
coordinator-deployment-1671042418    0         0         0         348d
coordinator-deployment-1785484075    0         0         0         1y
coordinator-deployment-1801327955    0         0         0         360d
coordinator-deployment-1806374227    0         0         0         362d
coordinator-deployment-1858864149    0         0         0         1y
coordinator-deployment-1927566065    0         0         0         1y
coordinator-deployment-193140103     0         0         0         357d
coordinator-deployment-1935218100    0         0         0         362d
coordinator-deployment-2058753461    0         0         0         1y
coordinator-deployment-247742532     0         0         0         304d
coordinator-deployment-2602702297    0         0         0         363d
coordinator-deployment-2742228346    0         0         0         347d
coordinator-deployment-2792313594    0         0         0         1y
coordinator-deployment-3000636796    0         0         0         1y
coordinator-deployment-3018724732    0         0         0         1y
coordinator-deployment-3037861148    0         0         0         359d
coordinator-deployment-3095810750    0         0         0         1y
coordinator-deployment-3236291850    0         0         0         298d
coordinator-deployment-3275429246    0         0         0         352d
coordinator-deployment-3421967647    0         0         0         351d
coordinator-deployment-3493766804    0         0         0         1y
coordinator-deployment-3685946753    0         0         0         355d
coordinator-deployment-4074640772    0         0         0         361d
coordinator-deployment-410800841     0         0         0         1y
coordinator-deployment-4109702372    0         0         0         1y
coordinator-deployment-4227798309    0         0         0         363d
coordinator-deployment-54449fc9fc    0         0         0         219d
coordinator-deployment-54577864f4    0         0         0         81d
coordinator-deployment-55c6df6f      0         0         0         37d
coordinator-deployment-57bfbdf59c    0         0         0         283d
coordinator-deployment-581506474     0         0         0         353d
coordinator-deployment-59ff458bdd    0         0         0         61d
coordinator-deployment-5cb8c5c849    0         0         0         22d
coordinator-deployment-5cc59f758f    1         1         1         10d
coordinator-deployment-650003267     0         0         0         1y
coordinator-deployment-657fb9bd6f    0         0         0         159d

/cc @andybons @dmitshur @katiehockman