buildscripts: fix GAE dummy app deploy races (#4308)
buildscripts: fix GAE dummy app deploy races (#4308)

This fixes a race where we can race with another deploy of the
'dummy' version.

Also, when we delete old versions, only delete versions beginning with
'kokoro'. This would make races even less likely because the
'dummy-default' should normally stick around forever.

This fixes this failure:
https://source.cloud.google.com/results/invocations/6b24aed4-7dc3-490a-87ea-c870d1435184/targets/grpc%2Fjava%2Fpresubmit%2Fgae-interop/log
stanley-cheung/grpc-java,ejona86/grpc-java,elandau/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,carl-mastrangelo/grpc-java,stanley-cheung/grpc-java,elandau/grpc-java,carl-mastrangelo/grpc-java,grpc/grpc-java,zhangkun83/grpc-java,grpc/grpc-java,ejona86/grpc-java,elandau/grpc-java,zhangkun83/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,zhangkun83/grpc-java,elandau/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,zhangkun83/grpc-java,carl-mastrangelo/grpc-java,ejona86/grpc-java