Bug 2109. Fix VIM info DB update after vertical scaling, migrate and operate

Fix the path of the VDU record to update.
Fix unexpected keyword argument 'disk_over_commit' in live_migrate method because of Nova microversion change (Bug 2262)
Handle the OS-EXT-SRV-ATTR:host KeyError as this key is raised if the VIM account does not have admin credentials
Expand unit tests for vertical_scale, migrate and operate task creations

Change-Id: Ic7c1fe639fc0b8f917c46c6fe61fd4089a6970b4
Signed-off-by: Patricia Reinoso <patricia.reinoso@canonical.com>