Use OpenSSL module instead of binary to generate certs

This saves us one dependency and saves us from complicated handling of
external files if we need key and certificate separated from each other.

At the same time, the number of bits used for RSA keys is increased from
1024 to 2048.

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