Allow vm to boot without l3 address(subnet)

Sep-12-2016(armax): landed.

Mar-3-2106(armax): Moved to Newton. Please ensure you re-submit spec if necessary.

Jan-18(mangelajo): I've asked to document in a devref, and change the proposed limit/limited portsec semantics to make it more natural, there were contradictions in the proposed one: https://review.openstack.org/#/c/250036/16//COMMIT_MSG

Dec-8 (mangelajo): Nova spec didn't get approved, neither code did:
            nova spec: https://review.openstack.org/#/c/239276/
                     code: https://review.openstack.org/#/c/202882/
            neutron code: https://review.openstack.org/#/q/topic:bp/vm-without-l3-address+project:openstack/neutron,n,z
            armax will ask for an exception while we get neutron patches in shape.

October-29 (armax): Kevinbenton volunteered to be an extra pair of eyes.

October-20 (mangelajo): As per conversation in https://review.openstack.org/#/c/202882/ , this needs a nova spec. Has it been submitted?, please be aware that nova deadlines are much tighter.

September-8 (mestery): Welcome to Mitaka!

August-26 (mestery): Are we still waiting on patches for this BP? The one that landed below indicates a partial implementation.

August-31 (armax): Change https://review.openstack.org/#/c/218470/ was a revert of
     https://review.openstack.org/#/c/200809/
The implementation strategy needs to be revised to avoid security vulnerabilities.

Gerrit topic: https://review.openstack.org/#q,topic:bp/vm-without-l3-address,n,z

Addressed by: https://review.openstack.org/183125
    allow unaddressed port(without l3 address, subnet) and to boot VM with it

Addressed by: https://review.openstack.org/200809
    Add support for unaddressed port

Gerrit topic: https://review.openstack.org/#q,topic:bug/1487598,n,z

Addressed by: https://review.openstack.org/218710
    Merge remote-tracking branch 'origin/master' into merge-branch

Addressed by: https://review.openstack.org/220811
    Add support for unaddressed port

Addressed by: https://review.openstack.org/241968
    Add support for unaddressed port

Addressed by: https://review.openstack.org/248487
    Create unaddressed port from network with subnet

Addressed by: https://review.openstack.org/250036
    Add new limit_portsec_domain attr for port

Addressed by: https://review.openstack.org/361455
    Mark unaddressed ports with none in ip_allocation field

Addressed by: https://review.openstack.org/360858
    Refactor _ipam_get_subnets