Implement IP address allocation.

Implement IP address allocation.

This fixes bug 1008029.

The allocation is done as follows (according to the value of
port['fixed_ips']):
- api_router.ATTR_NOT_SPECIFIED - Quantum will generate an IP address.
If a v4 subnet is defined then a v4 address will be generated. If
a v6 subnet is defined then a v6 address will be generated. If both are
defined then both v4 and v6 addresses will be generated.
- user configuration which may contain a list of the following:
   - ip_address - the specific IP address will be generated
   - subnet_id - an IP address from the subnet will be generated

Change-Id: I3bb1b83b8824364b37dbecfa140331c4a1fd2762