Add logging to journal for systemd

[stephan242]

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Right now no logs are visible for a boundary instance run via systemd. This PR adds 2 lines to the unit file to log stdout and stderr to journal, so that the logs can be seen with e.g.

journalctl -u [email protected]

That is, if one manually implements the PR https://github.com/hashicorp/boundary/pull/2268 which btw really should be merged asap, since right now boundary via systemd is essentially not working, unless one puts the config file one wants to run to /etc/boundary.d/.hcl, which would be a dangerous botch job.