objectify the gerrit event for our purposes

instead of passing around complex data structures, create an
event object for our purposes that means we can pass around the
payload relevant to us. Simplifies some things, and will make
adding build_uuid tighter.

Change-Id: I8172b25ae3c60e38d63cf7f4d8a0f6c854bae766
Merge "objectify the gerrit event for our purposes"