bootstrap: Add new function to create cluster certs and keys

The code to generate cluster certificates, keys and secrets is currently
spread over several places. It makes sense to move it to a separate
function as we want to provide the user with the ability to automatically
replace all cluster certificates and keys.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>