Add support for setting log level on webhook reporter

[Blake Rouse]

example config:

        'reporting': {
            'maas': {
                'type': 'webhook',
                'endpoint': absolute_reverse(
                    'metadata-status', args=[node.system_id],
                    base_url=base_url),
                'consumer_key': token.consumer.key,
                'token_key': token.key,
                'token_secret': token.secret,
                'level': 'INFO', # <--- breaks cloud-init
            }
        }

[ 22.612772] cloud-init[987]: 2017-02-14 20:34:37,062 - util.py[WARNING]: failed stage init
[ 22.619693] cloud-init[987]: failed run of stage init
[ 22.620822] cloud-init[987]: ------------------------------------------------------------
[ 22.621872] cloud-init[987]: Traceback (most recent call last):
[ 22.624257] cloud-init[987]: File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 524, in status_wrapper
[ 22.625446] cloud-init[987]: ret = functor(name, args)
[ 22.628192] cloud-init[987]: File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 197, in main_init
[ 22.629186] cloud-init[987]: apply_reporting_cfg(init.cfg)
[ 22.632254] cloud-init[987]: File "/usr/lib/python3/dist-packages/cloudinit/cmd/main.py", line 140, in apply_reporting_cfg
[ 22.633267] cloud-init[987]: reporting.update_configuration(cfg.get('reporting'))
[ 22.636398] cloud-init[987]: File "/usr/lib/python3/dist-packages/cloudinit/reporting/__init__.py", line 35, in update_configuration
[ 22.637576] cloud-init[987]: instance = cls(**handler_config)
[ 22.640352] cloud-init[987]: TypeError: __init__() got an unexpected keyword argument 'level'
[ 22.641405] cloud-init[987]: ------------------------------------------------------------

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/2807