Add trace logging to allocate_fixed_ip

The address is being logged as None in some cases
that are failing in grenade jobs so this adds more
trace logging to the base network manager's
allocate_fixed_ip method so we can see which paths
are being taken in the code and what the outputs
are.

Change-Id: I37de4b3bbb9e51b57eb4d048e05fc00382eed23d
Related-Bug: #1349617
Merge "Add trace logging to allocate_fixed_ip"