Standard servers

Be able to set default servers on system config, so they are used as default for new clients and new services created by admin.

Be able to set default server on templates, so they are set to clients when a template is set, and they can be changed for every client using template when template is changed. If template doesn't define a default server, behaviour is not changed and default server can be set on every client.

It implements FS#1100 from bugtracker