engine: Removed unwanted exception logging

Gluster task monitoring checks periodically for
gluster tasks list from the cluster, and if there
are no operational hosts in the cluster, an
exception is logged.
Removing the noise from this logging as it
confuses the user.

Change-Id: I9b24fe99a1cb66155a9ff24f149a20db32e76307
Bug-Url: https://bugzilla.redhat.com/1138128
Signed-off-by: Sahina Bose <sabose@redhat.com>
(cherry picked from commit be0f7e8f506cec907f91cd2a393abdf5663594b6)