Allow redis_namespace to be configured also in file stanzas

This commits allow to specify redis namespaces also in file stanzas, using [beaver] global config as default.

I'm not really sure it will use default namespace as default. Looking at the code, it seems that the get_field method returns the section default for redis_namespace, that is set to null string (previous commit). Maybe I completely misread the code, but it seems that removing the redis_namespace from _section_defaults will cause a KeyError in get_field. We can address this using a try: except KeyError approach or  modifying somewhat the get_field method to manage null string case, but I'm not sure this will be as straightforward as the extra if is.
What do you think?