Ensure combination alarms can be evaluated

Fixes bug 1237589

Previously the retrieval of state for the constituent alarms always
failed for combination alarms, due to an incorrect attribute of
ceilometerclient being used. As a result combination alarms could
never be successfully evaluated.

Now the correct pluralized attribute is used.

Change-Id: Id58a5d185854367e5a08011048774ab5aed93d4e
(cherry picked from commit f44e292e7a16d09db36d8b2f9223e3d54202f8d2)
Merge "Ensure combination alarms can be evaluated" into milestone-proposed