Code review comment for ~afreiberger/prometheus-openstack-exporter-snap:log-level-fix

When upgrading to new P-O-E snap, if the log_level is not set in the config file (such as is the case with prior p-o-e charm revisions prior to this commit [1], the following traceback is seen and p-o-e fails to start:

Nov 14 12:13:40 loomis prometheus-openstack-exporter.prometheus-openstack-exporter[31476]: Traceback (most recent call last):
Nov 14 12:13:40 loomis prometheus-openstack-exporter.prometheus-openstack-exporter[31476]: File "/snap/prometheus-openstack-exporter/30/bin/prometheus-openstack-exporter", line 795, in <module>
Nov 14 12:13:40 loomis prometheus-openstack-exporter.prometheus-openstack-exporter[31476]: numeric_log_level = getattr(logging, config.get('log_level').upper(), None)
Nov 14 12:13:40 loomis prometheus-openstack-exporter.prometheus-openstack-exporter[31476]: AttributeError: 'NoneType' object has no attribute 'upper'

[1] https://git.launchpad.net/prometheus-openstack-exporter-charm/commit/templates/prometheus-openstack-exporter.yaml?id=f6b2f96dd0c4d4656de8384f1308fcac0fefa2ef