Option "verbose" from group "DEFAULT" is deprecated for removal.
Its value may be silently ignored in the future.
If this option is not set explicitly, there is no such warning.
Furthermore, the default value of verbose is true, so there is no
need to set this value in config files.
And we use oslo_config.generator to generate a sample file.
We'd better fix the doc not encourage user to set verbose=True.
Change-Id: Ie9a68e511425d34cbce04e416f258db3064d0479
dac458af56