Update Logging option for the setting log level

In order to be able to more easily debug bmcweb related issue,
a new meson option is added to set a specific logging level
Which generates the targeted logging traces rather than all of
debug traces.

The current option -Dbmcweb-logging which can be either
disabled or enabled is changed to  allow  to set the log level
for the specific level traces (e.g. error or critical traces)
to be written to the journal.

     -Dbmcweb-logging=<log-level>

where <log-level> can be disabled, enabled, debug, info,
      warning, error, or critical.

- `disabled`: Turns off all bmcweb log traces.
- `enabled` : treated as `debug`
- Other option can be described in
  [Logging Levels](DEVELOPING.md).

For an example, to enable only 'error', 'critical' log
entries, bmcweb can be built with

    -Dbmcweb-logging=error

Testing:
- Verified that only the specific logs (e.g. error and
  critical logs) were displayed by compiling bmcweb with the
  specific bmcweb-logging level.

Change-Id: I522ca26700ea420fee1a5cf688d3e8c6661f2f55
Signed-off-by: Myung Bae <myungbae@us.ibm.com>