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
Merge "Ensure combination alarms can be evaluated"