Raise 404 when instance not found in admin_actions API

The pause/unpause/suspend/resume/reset_network admin action APIs do not
handle the case where the instance in the request is not found and they
raise HTTPUnprocessableEntity (code 422) instead. There are several
other operations in the same API which handle the 404 case and translate
it correctly, this patch cleans up the rest.

Closes-Bug: #1204999

Change-Id: If54d6be99db55b4f99da11fe75c14bf21685e809
Merge "Raise 404 when instance not found in admin_actions API"