MEDIUM: Lower priority of email alerts for log-health-checks messages

MEDIUM: Lower priority of email alerts for log-health-checks messages

Lower the priority of email alerts for log-health-checks messages from
LOG_NOTICE to LOG_INFO.

This is to allow set-ups with log-health-checks enabled to disable email
for health check state changes while leaving other email alerts enabled.

In order for email alerts to be sent for health check state changes
"log-health-checks" needs to be set and "email-alert level" needs to be 'info'
or lower. "email-alert mailers" and "email-alert to" settings are also
required in order for any email alerts to be sent.

A follow-up patch will document the above.

Signed-off-by: Simon Horman <horms@verge.net.au>