Allow alarm-threshold-update to upate generic attributes

Fixes bug 1253989

Previously, generic (i.e. non-threshold-related) alarm attributes could
not be updated with the alarm-threshold-update command. An attempt to
do so failed semi-silently when a non-existent dict key was referenced.

Now, all alarm attributes can be updated with this command.

Change-Id: Iba3f21de879fb853575dcec1730de7873eab8afd
Merge "Allow alarm-threshold-update to upate generic attributes"