Automatic merge from submit-queue (batch tested with PRs 40873, 40948, 39580, 41065, 40815)

Default target storage in etcd

To make etcd v2->v3 upgrade work correctly, we need to correctly set the "TARGET_STORAGE" env var. Since in head we are defaulting to etcd v3, this PR is defaulting also that env var to etcd3, so that by default upgrade works fine.

@fgrzadkowski @gmarek