Don't update API cell on get_nwinfo

Every call to get_instance_nw_info() updates an instance's info_cache.
The purpose of this is to make sure we're constantly healing the
info_cache table.  However, this causes a lot of unneeded updates
traveling up to the API cell.  Cells has its own periodic task that
heals all instance data.

Fixes bug 1180304

Conflicts:
	nova/network/api.py
	nova/tests/cells/test_cells_messaging.py

Change-Id: I07774712a6358999fd1fdd0f5a90c6f05e3d8d6e
(cherry picked from commit 4251bba3bd456f7ba1c7481bb84d839a5fafc757)
Merge "Don't update API cell on get_nwinfo" into stable/grizzly