Add Pluggable IPAM Backend Part 2

Add Pluggable IPAM Backend Part 2

Introduces new Pluggable IPAM backend.
IP/subnet allocation calls are sent to IPAM driver.
Calls to IPAM Driver are considered as call to third-party
environment, so if any action fails, rollback action is called.

Removes associate_neutron_subnet step from interface and reference
driver. It is not needed any more because foreign key relationship
between IPAM subnet and neutron subnet was removed.
So IPAM subnet can store id of neutron subnet,
which is not created yet.

For now only reference IPAM driver is available.

Temporarily disabled test_ipam_pluggable_backend from
gate-neutron-python34. Current patch adds executing
parts of test_db_base_plugin_v2, which is not py34 compatible yet.
Might be enabled back once 204791 is merged.

Partially-Implements: blueprint neutron-ipam

Change-Id: Ic18461cf19d2eebf8fad5535eee0eb034959800e