Configure SSL CA Cert in Dovecot

Currently, the SSL ca (bundle) SSL cert is not configured in dovecot.conf file. This may lead to connection issues with some older clients. We should add the line:

ssl_ca = </usr/local/ispconfig/interface/ssl/ispserver.bundle

in dovecot.conf, when the file /usr/local/ispconfig/interface/ssl/ispserver.bundle exists on the server.