Add validation for nova network ranges.

This adds validation for Public Floating Range and
Private Fixed Range.

In addition to the 'required' validation, this adds the following:
1) check for proper IP addr format
2) make sure the param specifies a *range* rather than a single address
3) check for range overlap by comparing the beginning and end of
   each range with the other range param.
Merge pull request #207 from sseago/nova-network-range-validation

Add validation for nova network ranges.