staged_rollouts: Push v1.57.0 to 15% of instances #317

Job	Run time
installation-test	29m 32s
update-test	28m 3s
update-postactivation-snapshot	2m 27s
install-postactivation-snapshot	3m 26s
cleanup (23981403404-install)	23s
cleanup (23981403404-update)	23s
cleanup-snapshots	13s
cleanup (23981403404-distupgrade)	17s
	1h 4m 44s

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