Remove requirement on presence of config file

The NipapConfig object required the presence of a configuration file. As
it is benficial to be able to load the NipapConfig without a
configuration file (for example when building docs using Sphinx), this
requirement has been removed.
Fix NipapConfig exception handling
Avoid trying to read config

If we don't have a configuration file specified.. don't try to read it.
Refactor xmlrpc to allow import without init

The xmlrpc module initiated an instance of the nipap backend and a few
other things which made it impossible to simply import the module
without first setting up all the prerequisites. Unittesting and similar
was made difficult.

This change moves everything into a class so that nothing is
instantiated upon import and thus unittesting or things like generating
docs (by sphinx) is easier.
Merge branch 'xmlrpc-refactor'