NTP configuration for imaging worker

[sjones4]

The install guide Install and Configure the Imaging Service section should mention that an NTP server may need to be set for the imaging worker, e.g.:

# euctl services.imaging.worker.ntp_server=time.google.com

this is required if public NTP servers are not accessible.

We may also want to add a troubleshooting item for this under Imaging Worker similar to the existing item for ELB.

https://docs.eucalyptus.cloud/eucalyptus/4.4.5/index.html#install-guide/configure_imaging_service.html
https://docs.eucalyptus.cloud/eucalyptus/4.4.5/index.html#troubleshooting-guide/ts_elb.html
https://docs.eucalyptus.cloud/eucalyptus/4.4.5/index.html#install-guide/configure_load_balancer.html