Ensure combination alarm evaluator can be loaded

Fixes bug 1237578

Previously combination alarms were always skipped as the evaluator
was missing from the loadable stevedore extensions.

Change-Id: I9d164fb2a89ee9931fc032d93ad4a26ba6887f0b
(cherry picked from commit 61af83ddd832e63deb4b72fa11c7b18df7ae4702)
Merge "Ensure combination alarm evaluator can be loaded" into milestone-proposed