Decompose create_port and save_subnet

Decompose create_port and save_subnet

This commit is a preparation step for enabling pluggable ipam.
Some actions in create_port and save_subnet are specific for
non pluggable ipam implementation.

- create_port
Moved allocation ips for port and storing results into separate method
_allocate_ips_for_port_and_store.
Moved to ipam_non_pluggable_backend, since pluggable implementation will
be different due to rollback on failure logic included.

- save_subnet
Moved saving allocation pools into new method _save_allocation_pools.
Moved to ipam_non_pluggable_backend, since pluggable ipam implementation
does not need to save IPAvailabilityRange (availability ranges are
maintained by ipam driver for pluggable case)

Partially-Implements: blueprint neutron-ipam

Change-Id: I4a3e5d7f3aa54630279d9589225a509c96ed2186