Factorize code for checking node daemon certificate

This code is going to be used by a new utility for setting up the node
daemon. Unit tests are updated/added.

Additionally, the certificate and key stored in “server.pem” are
verified, too.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>