Bug 534704 (RHQ-1475)

Currently, there is no way to check if a potential new plugin-config is valid or not. When a user makes a bad change, the connection to the managed resource will break and the user will see a red or yellow alert box.

We should split this into two parts:
- first an optional verify step that the plugin writer can overwrite -- as default it returns "valid"
  - even for valid configurations it should be possible to return warning or information messages 
- second if the config was considered valid in the first step the actual activation of this new config.

This is no trivial request, as it will include changes in server, plugin-api, gui and plugins.