Read conf sections and override flags

The following PR allows for overriding flags from subcommands in ceph-deploy when defined in the configuration.

To avoid overwriting flags from other subcommands e.g. if you defined a `foo` value for both the `new` and the `mon` subcommand with different values, it makes sure it only sets values that match both the section and the subcommand.

Documentation already covered this so it will not get updated.

Also some functions where renamed so that we can rely on `func.__name__` and map that to the config section