Convert address to str in fixed_ip_obj.associate

A caller of associate may pass in a netaddr.IPAddress argument which
will currently fail when it gets to sqlalchemy.  Following the
convention of other methods the netaddr.IPAddress will be cast to a
string.

Change-Id: I67c1876b447217b37104660ba87ed967f477b863
Closes-bug: 1317668
Merge "Convert address to str in fixed_ip_obj.associate"