Problem/Motivation
Currently it is not possible to include !severity as part of the logged message by syslog. This information is available to be logged, but there is no !severity placeholder.
Proposed resolution
Add !severity as one of the available placeholders.
To avoid break BC compatibility, do not include by default in syslog config.
The work in #1748410 was committed to D8. A question is whether, as with that issue, there needs to be a D7 port of #2874069 so that tests can be added.
