move os_vif.initialize() to nova-compute start

move os_vif.initialize() to nova-compute start

os_vif.initialize() was previously executed during module load. This
means it was entirely possible that it was run before things like
logging were actually set up in the expected way. Move this back into
execution time instead of load time to ensure that logging is actually
setup.

Changes need to be made to tests which make assumptions about os_vif
objects to manually initialize os_vif when it will be used. os_vif
objects can't be created until it is initialized, so some delayed
object creation is also done in test_vif.py.

Closes-Bug: #1615676

Change-Id: I89fe5c5b3d762f3a3238b587685df85d15ee56c4