engine: Change gluster brick status to Unknown

When a host goes to non responsive state, change the
gluster brick's status on the host to Unknown

Change-Id: I80f0e581b86a34b649343a931eaccf8e41b8e275
Bug-Url: https://bugzilla.redhat.com/1021441
Signed-off-by: Sahina Bose <sabose@redhat.com>