Reintroduce fname kwarg in configmanager

In [f04f409], the configmanager's __init__ lost its fname kwarg, which
allows to pass the name of the config file to read (or write).
Without it the only way to programmatically specify this file name is to
use the OPENERP_SERVER environment variable, which may be unpractical and/or
subject to renaming.

External tools (such as the buildout recipe) may need to pass this file in a
clean way.