Update instance's cell_name in API cell.

It was accidentally being removed before updating the DB due to code
being moved and not updated.  The test for the method was broken, too,
allowing this bug to sneak in.

Fixes bug 1098093

Change-Id: I05c00b8b100568b39afb8ee0ca7ee94d4dd97c0a
Merge "Update instance's cell_name in API cell."