Add tag as SYSLOG_IDENTIFIER for journald log driver

[rjhowe]

- What I did
Added SYSLOG_IDENTIFIER when using docker journald log driver. When sending logs from journal to rsyslog programname will be set to the SYSLOG_IDENTIFIER and not defaulted to getting set to journal.

- How I did it
moby@ae557f9
https://docs.docker.com/config/containers/logging/journald/

- How to verify it
Run:

# docker run --name logging-test --entrypoint=/bin/echo registry.access.redhat.com/rhel7 "Hello Logs"
# journalctl CONTAINER_NAME=logging-test -o verbose

Then look for SYSLOG_IDENTIFIER being set to the same value as CONTAINER_TAG

- Description for the changelog
Added tag as SYSLOG_IDENTIFIER for journald log driver

[rjhowe]

Issue discussed in the following bug: https://bugzilla.redhat.com/show_bug.cgi?id=1527918