Proxy settings analyzer (and preflight example)

[xavpaice]

In many situations we see installations not working, or failing to install, because the proxy settings on site are incorrect.

We need to highlight during a preflight:

• if proxy is in use, can it be used to reach a particular location
• if proxy is used, does NO_PROXY include the node address, and/or the k8s API address

[xavpaice]

replicatedhq/troubleshoot#1302 to create the collector/analyzer if it can't be done with existing tools