Auto allocate gateway_ip even for SLAAC subnets

For a SLAAC subnet that is created without specifying the gateway_ip,
Neutron currently allocates (If0c48a7287a828eef4a0f0b0859d4f898d2937bd)
the gateway_ip at a later stage (i.e., neutron router_interface_add).
In order to keep the API consistent between IPv4 and IPv6, it is
recommended to allocate the gateway_ip during subnet_create stage itself.

Closes-Bug: #1394112
Change-Id: I965232930502c21b605fe360bb138bb6ea73d2b0
Have L3 agent catch the correct exception

L3 agent imports the processutils module to catch exceptions that
wouldn't ever be thrown because the underlying execute() being
called is the one from neutron.agent.linux.utils which raises a
RuntimeError on failure.

Also, processutils is now part of oslo.concurrency. So when we
actually start using it, we'll use it from there.

Closes-Bug: 1401626
Change-Id: I43874e1b63a0ba7b01415cafe0538f4343057066
Merge "Have L3 agent catch the correct exception"
Merge "Auto allocate gateway_ip even for SLAAC subnets"
Merge branch 'master' of https://github.com/openstack/quantum