CLOUDSTACK-7415. Host remains in Alert after vCenter restart.
Management server PingTask should update PingMap entry for an agent only if it is already present in the Management Server's PingMap.

(cherry picked from commit 8ce6eba549bcd3fa007aaf10a29c3a2fef9ffaaa)
Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
(cherry picked from commit 59ce63918e227f93d64642e881551c25738de3b3)