validate node_id

[Sunz3r]

if the node_id contains invalid characters then rrd-update interrupt the backend-script

[tcatm]

Thanks!

Could you

• refactor the else branch to an if branch to make it clearer what's being tested?
• add a note to the readme stating what is considered a valid node id so people don't have to read the code?
• write a more descriptive commit message stating what the problem is and how it was addressed? This should also include a short description of what makes a node id valid.

[rubo77]

@Sunz3r are you still planing to get this straight?