[BZ 1022943] Ability to trigger update of plugins on agents from UI and API

This provides basic support for triggering updates of plugins on agents
by calling server-side APIs (remote API/CLI/REST)  or GUI.

The deployment methods are synchronous, meaning that they only return once a plugin is fully deployed.

Updating the agents with the latest version of plugins is an asynchronous operation with a possibility to track progress of it (a mere boolean - is finished or not?).