allow for users to define the set of registries in `configure-docker`

[dekkagaijin]

• Let users specify the set of registries via configure-docker --registries="foo.gcr.io, bar.gcr.io, baz.gcr.io"
  • Default: "gcr.io, us.gcr.io, eu.gcr.io, asia.gcr.io, marketplace.gcr.io"

Fixes https://github.com/GoogleCloudPlatform/docker-credential-gcr/issues/49