Step CA not renewing cert after waking from sleep on OSX

[dopey]

Steps to reproduce:

1. Set default cert validity duration to an appropriately small value; e.g. 10 mins

2. Start the CA, put the machine to sleep before the cert has a chance to renew

3. Wake the laptop after the cert had sufficient time to renew.

4. Make a step ca health request and witness failure due to expired cert.

[maraino]

Fixed with #11