Track allocation_pools in SubnetRequest

Track allocation_pools in SubnetRequest

To keep pluggable and non-pluggable ipam implementation consistent
non-pluggable one has to be switched to track allocation_pools and
gateway_ip using SubnetRequests.
SubnetRequest requires allocation_pools to be list of IPRanges.
Previously allocation_pools were tracked as list of dicts.
So allocation_pools generating and validating was moved before
SubnetRequest is created.

Partially-Implements: blueprint neutron-ipam

Change-Id: I8d2fec3013b302db202121f946b53a0610ae8321