net-im/ejabberd: Don't try to generate cert if it is already set up

net-im/ejabberd: Don't try to generate cert if it is already set up

The ebuild used to always try to generate a certificate. ssl-cert eclass
function warns if certificate in a specified location already exists,
but that only clutters logs. When user has set different path to a
certificate, then eclass used to generate a self-signed cert which
wouldn't be used at all. Check if certificate set by user exists before
generating a new one.

Package-Manager: portage-2.3.0_rc1