Revert "Convert address to str in fixed_ip_obj.associate"

This reverts commit 5268c1136ed09346d13b28b42626c0b302e3abb2.

A change was made that breaks RPC compatibility since the actual
remotable method was changed to the underscored version.

Change-Id: I2589e914b5141fdea0738d04b4a8e14e349580a0
Merge "Revert "Convert address to str in fixed_ip_obj.associate""