Add request factory for pluggable IPAM

Add request factory for pluggable IPAM

Pluggable IPAM implementation requires separation between requesting
address/subnet and it's actual allocation, which can happen on
third-party IPAM servers. Request factory stands for simplifying
building right request from input.

Added AddressRequestFactory and SubnetRequestFactory.

AddressRequestFactory creates instance of AnyAddressRequest or
SpecificAddressRequest depending on presence of ip address in input.
SubnetRequestFactory creates instance of AnySubnetRequest or
SpecificSubnetRequest depending on input.

get_subnet_request_factory and get_address_request_factory can be
redefined on driver level to use custom request factories.

Partially-Implements: blueprint neutron-ipam

Change-Id: Iedc0cfa326d60810099148f0ef8a1edac9e8aa12