Fix Implicit use of 'generic-host' template in objectbrowser/add/host

Fixes #459

This patch fixes an issue in objectbrowser/add/host when
generic-host is implicitly selected as a template without
checking if there is a template called generic-host.

This patch makes the following changes:
* Make template used e.g. "generic-host" configurable from settings
* Change objectbrowser/add so that if the specified template does not
  exist, then empty string is used as an initial value for template
* Same patch applied for services and contacts as well.