mongo: remove _id in inserted alarm changes

This change ensures that mongodb driver doesn't modify the alarm changes
dictionary when we insert a new alarm changes. To avoid the insertion
of the internal '_id' of mongodb.

Change-Id: I9bcd643c9dc55449141cee94ef6826784e81e4e5
Merge "mongo: remove _id in inserted alarm changes"
oslo.messaging context must be a dict

oslo.messaging assumes the context is a dict not a RequestContext
and it assumes the payload in json serializable.

This patch ensures this.

Also it removes oslo.messaging mock on some tests and use real oslo.messaging
library with the fake driver.

Change-Id: Ie3c6083bbc4ec83de28e42bb10e7c50c7e135070
Closes-bug: #1275771
Closes-bug: #1317290