Splits server startup into prepare/start phases. Server's agent is now
started after server is prepared, but before it is started. This allows
us to properly bootstrap the executor before starting server components,
and use the executor to provide a shared channel to wait on apiserver
readiness.

This allows us to replace four separate callers of WaitForAPIServerReady
with reads from a common ready channel.
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>