The default 10 seconds health polling interval is too frequent for most of
the cases. Now it has been changed to 60s. A new config
`health_polling_interval` is added to make the interval configurable.
Cloud admin can totally disable the health polling by set a negative value
for the config.

Task: 39795
Story: 2007683

Change-Id: Iad30487b8c119e94ee21d75f53fb86eb23dff365