webgui: add optional 'default-operation' into edit-config request
webgui: add optional 'default-operation' into edit-config request

This will allow us to replace the whole configuration subtree in
datastore. Therefore, we can support ordered-by user lists on sections
(entries in top-menu), where we send form with the whole subtree.

- refs #950

CESNET/Netopeer-GUI,CESNET/Netopeer-GUI,CESNET/Netopeer-GUI,CESNET/Netopeer-GUI,CESNET/Netopeer-GUI,CESNET/Netopeer-GUI