Change the server-url config to include the base URL up through the context path

[kierarad]

Essentially, we want change and document the expectations around setting server-url to include the base URL up through the context path, in case users have changed this from the default /go:

# example
gocd config server-url https://my.gocd.com/go

The preflight api call will need to be updated to account for this change.

Documentation

We should describe what we mean by "base URL" and maybe give an example of what "context path" means, noting that by default this is /go.

[kierarad]

Fixed in 362d024