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.

Hmmm, it should probably call a `.get()` with a default of None. 

Ah I think you're right, it won't fallback to the global namespace. It does seem odd to me that here we'd fallback, but in other places we won't fallback to the global beaver config. Hmmm.

Leave it as is I guess.