filepromoter: Build unauthenticated client for source filestores

[justaugustus]

What type of PR is this?

/kind bug

What this PR does / why we need it:

(Part of kubernetes/k8s.io#2624, #413.)

• filepromoter: Build unauthenticated client for source filestores

  Source filestores are (supposed to be) world-readable, so there is
  no need to require an authenticated client for them.

  This is a workaround for the fact that service accounts are stored
  as part of filestore, when ergonomically, they should probably be
  at the top-level of a root promotion manifest. To be fixed in a
  future commit.

• kpromo: Build v0.2.4-1 image

Signed-off-by: Stephen Augustus [email protected]

/assign @puerco @ameukam @cpanato @Verolop
cc: @kubernetes-sigs/release-engineering

Which issue(s) this PR fixes:

Special notes for your reviewer:

Discovered in https://prow.k8s.io/view/gs/kubernetes-jenkins/logs/post-k8sio-file-promo/1437091170512539648, following the merge of kubernetes/k8s.io#2704:

********** START **********
level=info msg="processing destination \"gs://k8s-artifacts-prod/binaries/kops/\""
level=fatal msg="run `kpromo run files`: error building operations: error building promotion operations for \"gs://k8s-artifacts-prod/binaries/kops/\": cannot execute a production file promotion without a service account" 

Does this PR introduce a user-facing change?

- filepromoter: Build unauthenticated client for source filestores
  
  Source filestores are (supposed to be) world-readable, so there is
  no need to require an authenticated client for them.

  This is a workaround for the fact that service accounts are stored
  as part of `filestore`, when ergonomically, they should probably be
  at the top-level of a root promotion manifest. To be fixed in a
  future commit.

- kpromo: Build v0.2.4-1 image

[k8s-ci-robot]

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