hide clean_exit() from public API

The clean_exit() function should not be clearly visible in cloubed
public API since it must be and stay useless for external programs. Its
purpose is only to be called by atexit()
avoid _clean_exit() creates instance of Cloubed

If the instance of Cloubed has not been created before,
cloubed._clean_exit() should avoid creating a new useless one. This is
done by extending Singleton metaclass with the new method initialized().
avoid nonexistent Configuration.get_file_path()

Due to various code displacement, method get_file_path() moved from
Configuration class to ConfigurationLoader. Therefore this method does
not exist on object Cloubed._conf.
add all needed internal method to return xml

Cloubed generates XML description of resources to give them at the
libvirt library. This may be helpful sometimes to test manually these
XML input against libvirt through virsh.
add xml action on script

It could be used to print XML input generated by Cloubed for libvirt
for each resource.

Fixes #19.