Convert Events to separate action/status

First step in decoupling action/status in Resource

This adjusts the Event DB columns, model and class to split
action/status, but in order to be backwards compatible we set the action
to None, which means we can do the right thing in the CFN API, and
resource_status still maps correctly to what is expected in
python-heatclient (which will need to change after the Resource
action/status split happens)

Change-Id: I261d47d2a3fdb3cdc2a8aaf6c159763f4636f633