Fix driver loading

Driver loading is broken due to a circular import.
This patch fixes it by moving the instantiation of stevedore's
NameDispatchExtensionManager out of parse time so that it isn't
re-invoked each time stevedore imports another driver module.

Fixes bug 1212030

Change-Id: I59c8705d1fbcc41b0a34ffb203ca907dedb15df5