Remove default setting of network_size

The network_size parameter should be left unspecified.  Unless we add
this as a user specified option in the UI, we should leave it
unspecified so that Nova will calculate the best value given other
parameters.

Also update the TODO to reflect what's left to do for num_networks.
Merge pull request #184 from russellb/remove_network_size

Remove default setting of network_size