Collector Sidecar should exit if node ID is empty

[joschi]

Problem description

The Collector Sidecar tries to generate a unique node ID, use a given unique node ID, or fallback to the hostname to identify itself with Graylog.

Under some (unclear?) circumstances, the node ID might be empty or blank which leads to errors when requesting configuration from Graylog.

As a part of the bootstrap checks, the Collector Sidecar should check for invalid node IDs and exit with a non-zero exit code if the node ID is invalid.

https://github.com/Graylog2/collector-sidecar/blob/0.1.6/context/context.go#L69-L76
https://github.com/Graylog2/collector-sidecar/blob/1.0.0-alpha.2/context/context.go#L73-L81

Refs https://community.graylog.org/t/unable-to-run-collector-sidecar/5883

Environment

• Sidecar Version: 0.16 (probably also 1.0.0-alpha.2)
• Graylog Version: 2.4.5 (probably also 3.0.0-XXX)