collisions of duplicate ipvlan and macvlan network IDs during createNetwork

PR #2055 resolves the issue shown below, where an existing copy of a network has not been removed on Docker Engine shutdown, and still persists when the config-only network is created once again on a subsequent createNetwork call

existing network config = (*ipvlan.configuration)(0xc420322e80)({\n
  ID: (string) (len=25) \"hwqzu5i4kmrd2uzi34zpnpy1m\",\n
  Mtu: (int) 0,\n
  dbIndex: (uint64) 6,\n
  dbExists: (bool) true,\n
  Internal: (bool) false,\n
  Parent: (string) (len=4) \"eth0\",\n
  IpvlanMode: (string) (len=2) \"l2\",\n
  CreatedSlaveLink: (bool) false,\n
  Ipv4Subnets: ([]*ipvlan.ipv4Subnet) (len=1 cap=4) {\n
    (*ipvlan.ipv4Subnet)(0xc4206413a0)({\n
      SubnetIP: (string) (len=13) \"10.100.0.0/16\",\n
      GwIP: (string) (len=13) \"10.100.0.1/16\"\n
    })\n
  },\n
  Ipv6Subnets: ([]*ipvlan.ipv6Subnet) <nil>\n})\n

new network config = (*ipvlan.configuration)(0xc4203c5b80)({\n
  ID: (string) (len=25) \"hwqzu5i4kmrd2uzi34zpnpy1m\",\n
  Mtu: (int) 0,\n
  dbIndex: (uint64) 0,\n
  dbExists: (bool) false,\n
  Internal: (bool) false,\n
  Parent: (string) (len=4) \"eth0\",\n
  IpvlanMode: (string) (len=2) \"l2\",\n
  CreatedSlaveLink: (bool) false,\n
  Ipv4Subnets: ([]*ipvlan.ipv4Subnet) (len=1 cap=1) {\n
    (*ipvlan.ipv4Subnet)(0xc42149af00)({\n
      SubnetIP: (string) (len=13) \"10.100.0.0/16\",\n
      GwIP: (string) (len=13) \"10.100.0.1/16\"\n
    })\n
  },\n
  Ipv6Subnets: ([]*ipvlan.ipv6Subnet) <nil>\n})\n

The symptom of an existing and now-conflicting network is usually as follows, including showing up as a rejected task's error -

docker.err.log:time="2018-01-15T23:32:21.993891743Z" level=error msg="fatal task error" error="network di-hwqzu5i4kmrd is already using parent interface eth0

Start by reviewing PR #2055 and the createNetwork path for ipvlan and macvlan networks. Reproduce the Docker Engine shutdown and subsequent config-only network creation scenario described here, then check the rejected task error for a conflicting parent interface. Done means the stale network copy no longer causes duplicate network IDs or parent-interface conflicts.