Fix Hyper V instance conflicts

Fixes bug 1152368

If the hyperv driver attempts to boot an instance and a previous
instance with the same name was not properly removed, the boot will
fail due to an error creating root.vhd.  This only happens when
using CoW images.  This fix deletes the instance directory before
creating the new instance so that there will never be an old image
to interfere with creation of the new one.

This is most likely to be an issue in development and test
environments that toggle between versions of nova with no migration
path.

Change-Id: I21eab01590c426234b4b9a55abec4b2d2df0e168
Merge "Fix Hyper V instance conflicts"