core : No email notification sent when host recovers from previously reported condition

There is no option in event manager to subscribe
for 'Host is responsive' event.
Users can choose to receive email notifications
for several host conditions listed under "Host Event"
section of event notifications.
When a user has chosen a specific condition and such
condition occurs the user receives an email notification.
When such condition is resolved or disappears the user
does not receive any email notification alerting him/her
that the previously reported condition has been solved
is no longer present.

Fixes the case where vdsm is rebooted and host
comes back to Up status.

Change-Id: I2982b21f5d356847d994c6dcdf24e79e56479ecb
Bug-Url: https://bugzilla.redhat.com/1013631
Signed-off-by: Ravi Nori <rnori@redhat.com>