grizzly: nova periodic work needs doc update

[Tom Fifield]

In https://review.openstack.org/#/c/18618/ changes to the way periodic tasks work in nova were introduced. This involves changes to configuration options, so these areas of the manuals need to be updated:

default, text change:

 118 cfg.IntOpt("image_cache_manager_interval",
 default=2400,
  help='Number of seconds to wait between runs of the image ' cache manager'),

default, text change:

   cfg.IntOpt("running_deleted_instance_poll_interval",
  default=1800,
   help="Number of seconds to wait between runs of the cleanup " "task."),

new option:

70 cfg.BoolOpt('run_external_periodic_tasks',
   71 default=True,
   72 help=('Some periodic tasks can be run in a separate process. '
   73 'Should we run them here?')),
   74 ]

option replaced:

 52 cfg.IntOpt('periodic_interval', 52 cfg.BoolOpt('periodic_enable',
 53 default=60, 53 default=True,
 54 help='seconds between running periodic tasks'), 54 help='enable periodic tasks'),

[Tom Fifield (fifieldt)]

A paragraph should be added in the nova description in doc/src/docbkx/common/getstart.xml

[Michael Still (mikal)]

I feel like this is too technical for the getting started page. I'm going to add something to the operators guide instead.

[Michael Still (mikal)]

https://review.openstack.org/#/c/45912/ didn't get linked properly because I can't have nice things.