Ensure info cache updates don't overwhelm cells

Cells does its own healing of the info cache outside
of get_instance_nw_info, so we have double healing
happening if info_cache_cells_update is called for every
get_instance_nw_info. This also overwhelms cells'
ability to keep the global db updated at scale. Similar
functionality exists in nova/network/api.py.

Change-Id: I58f681f3674d826e3dce4fa956e2eb0e4c60f82b
Merge "Ensure info cache updates don't overwhelm cells"