19181 - In the rpc app discovery should happen after validation

The RPC application has various different modes of behaviour like
receiving JSON in, reading txt files for node lists or just simply
forcing verbose discovery.

We therefore need to do a specific discover before calling the remote
agents which has the undesired side effect of taking 2 or 3 seconds
before you get an error message if you supplied incorrect arguemnts on
the CLI.

This commit adds a specific validation call before doing any discovery,
the validation will cause early errors if any