test: Refactor flynn host bootstrap commands

Signed-off-by: Lewis Marshall <lewis@lmars.net>
test: Set random host ids

Booted instances all have the same hostname, leading to them all having
the same host ID and ListHosts() only containing one of them (because it
returns a map[string]host.Host)

Signed-off-by: Lewis Marshall <lewis@lmars.net>