loglevel configuration precedence and visibility

After experimenting with the log level configuration settings and options, I think the following rules should be implementing for the logging config

1. There is an implicit global setting of "ERROR". When a component starts if
   the global values is not in the KV store it should be written to the KV store.
2. The global log level can be set, but cannot be cleared.
3. When a component starts and if a log level is not set for that component in the
   KV store the component should write its log level as set via the command
   line option to the KV store as its current log level setting.
4. When a component starts and if a log level setting for that component exists in the KV
   store the component should set its log level to the saved setting
   overriding the command line option. This is only for a specific component
   setting and a component on startup should not "assume" the global setting over its command-line option setting.

This represents some significant changes from the original design but reflects practical experience with the API and configuration via the CLI. Likely the most significant change is that that voltha-lib-go is no longer just a "reader" of configuration information, but also a writer.