Revert "oslo.messaging context must be a dict"

This reverts commit 5df4cb53ae36ddb2a2495cc7aa9a1f3072133930.

The commit in question appears to have broken the notification agent
as an unintended side-effect. Reverting pending further investigation
as the notification agent is the more crucial component.

Related-to: #1317290
Change-Id: If9b1d638b60b7029c67f45e2e9feda62b9fd042a