Add threshold for nova services alerts

Could be configured through new variables:
 * services_failed_warning_threshold_percent
 * services_failed_critical_threshold_percent
 * computes_failed_warning_threshold_percent
 * computes_failed_critical_threshold_percent

Change-Id: I7cf5f00f4384776661dde5e07883449c37715ae2
Closes-Bug: PROD-15201
2 files changed