AlertGenerator: Fix NPE from CLOUDSTACK-3272

There was a commit 4b89a45e571a34dff0191d501089aa9b1046bc08 on master which was
added by "root@devcloud.apache.org" with no committer signature so I cannot tell
whom to reach. In the code it looks like they forgot to initialize the static
config dao variable which is causing NPE on master. The fix simply initializes
the s_configDao with injected configDao object.

Signed-off-by: Rohit Yadav <bhaisaab@apache.org>