certs: generate_csr: allow to set CN explicit

certs: generate_csr: allow to set CN explicit

Else, when used with ACME, the SAN is always sorted so we always get
the Subject Alternative Name sorting alphabetically first, which
doesn't necessarily has to be the "primary" domain. While this is
rather cosmetically (all SANs are equal) it could still result it
flapping CN when SANs and thus possibly the order changes, e.g., in
our CDN mirror pool. It also doesn't costs anything to allow control
over this, so why not..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>