r/fluent-bit: Set HOSTNAME environment variable

Fluent-bit does not have any native capability for setting a field with
the hostname of the machine, but it can set a field with the value of an
environment variable.  Thus, we can set the `HOSTNAME` environment
variable and then use that to set the field in the pipeline.

roles/fluent-bit/files/fluent-bit.service

@@ -10,6 +10,7 @@ StartLimitBurst=5
 Type=exec
 ExecStart=/usr/bin/fluent-bit -c /etc/fluent-bit/fluent-bit.yml -Y
 ExecReload=/bin/kill -HUP $MAINPID
+Environment=HOSTNAME=%H
 StateDirectory=fluent-bit
 Restart=always
 RestartSec=1