Use systemd logging levels

Systemd has an option[1] that allows it to interpret our log levels
directly.  This allows for journald to sort/filter/colorize our logs
better than it was able to previously.  Its indexes don't map perfectly
to bmcwebs, so come up with a constexpr lookup table to map the two
values across.

Tested: Enabled logging, and dumped journal logs.  Observed colorized
output.

[1] https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#SyslogLevelPrefix=

Change-Id: I7722ae86e114daec88709b68405498eeb8164c07
Signed-off-by: Ed Tanous <etanous@nvidia.com>
diff --git a/config/bmcweb.service.in b/config/bmcweb.service.in
index 040e519..07e2ad6 100644
--- a/config/bmcweb.service.in
+++ b/config/bmcweb.service.in
@@ -9,6 +9,7 @@
 ExecStart=@MESON_INSTALL_PREFIX@/libexec/bmcwebd
 Type=simple
 WorkingDirectory=/home/root
+SyslogLevelPrefix=true
 
 [Install]
 WantedBy=network.target