Always set commonName in X509 certificates

Due to the current switch of the RPC client to PycURL, a bug with newer
versions of libcurl surfaced. When the 'Subject' or 'Issuer' of
'server.pem' were empty, SSL handshake failed.

This patch changes the certificate generation functions such that they
always use "ganeti.example.com" as commonName (CN) for 'Subject' and
'Issuer'.

Signed-off-by: Manuel Franceschini <livewire@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>