dsinstance: Detect dynamic plugin support and restart server accordingly

With 389-ds-base 1.3.3. come the dynamic plugin support. We need to
restart the server right after modifying the schema, as the plugins
will be enabled at the point they are added (and not at the next
server restart).

Properly handle both situations in the installer.

https://fedorahosted.org/freeipa/ticket/4203