containerd: no keyring creation on backend.Create
containerd: no keyring creation on backend.Create

it turns out that the number of keyrings that one can have created it
quite limited for the needs of our workers.

it's also very unlikely that one would be using keyring for the builds
they make on concourse anyway (not something we ever promised to
isolate).

http://man7.org/linux/man-pages/man7/keyrings.7.html

Signed-off-by: Ciro S. Costa <4f5054ab2db051d03adf9068233a850c82b24437@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse