Add WaitDelay to ensure cmd.Wait() returns in a reasonable timeframe if
the goroutines that cmd.Start() uses to copy Stdin/Stdout/Stderr are
blocked when copying due to a sub-subprocess holding onto them. Read
more details in these issues:

- golang/go#23019
- golang/go#50436

This isn't the original intent of kill-delay, but it seems reasonable
to reuse it in this context.

Fixes canonical#149

It's unlikely to be needed here, but it won't hurt either. Use a
smaller timeout (1s) as these are intended for short-running commands.

Our style is to start with an uppercase letter (for logs, not errors)
and to use "Cannot X" rather than "Error Xing".

Also factor out setupEmptyServiceManager as the same code is used in 7
places.