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: #1402407
Closes-Bug: #1394112
Partial-Bug: #1377985
Change-Id: I965232930502c21b605fe360bb138bb6ea73d2b0
(cherry picked from commit 66dcb8b935a00e7f7566802d662ebb1f265eab1f)
Merge "Auto allocate gateway_ip even for SLAAC subnets" into stable/juno