Create container with duplicated name returns 400 instead of 409

[cezarsa]

Description
Creating containers with duplicated name used to return a 409 HTTP response, however, since 17.09 release it now returns a 400 response breaking compatibility for API users.

Steps to reproduce the issue:

1. Create a container docker create --name c1 busybox
2. Run curl -v -XPOST -H"Content-Type: application/json" -d'{"Image":"busybox"}' http://localhost:2375/containers/create?name=c1

Describe the results you received:

The HTTP response will be 400 Bad Request.

Describe the results you expected:

Prior to 17.09 the response would be 409 Conflict

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client:
 Version:      17.09.0-ce
 API version:  1.32
 Go version:   go1.8.3
 Git commit:   afdb6d4
 Built:        Tue Sep 26 22:42:18 2017
 OS/Arch:      linux/amd64

Server:
 Version:      17.09.0-ce
 API version:  1.32 (minimum version 1.12)
 Go version:   go1.8.3
 Git commit:   afdb6d4
 Built:        Tue Sep 26 22:40:56 2017
 OS/Arch:      linux/amd64
 Experimental: false

Output of docker info:

Containers: 7
 Running: 6
 Paused: 0
 Stopped: 1
Images: 18
Server Version: 17.09.0-ce
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 55
 Dirperm1 Supported: true
Logging Driver: syslog
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file logentries splunk syslog
Swarm: active
 NodeID: ug3ojsrrreyxeq63nbwyikv3f
 Is Manager: true
 ClusterID: n2efj7ylwed1onrdxy0sp4dp1
 Managers: 1
 Nodes: 1
 Orchestration:
  Task History Retention Limit: 5
 Raft:
  Snapshot Interval: 10000
  Number of Old Snapshots to Retain: 0
  Heartbeat Tick: 1
  Election Tick: 3
 Dispatcher:
  Heartbeat Period: 5 seconds
 CA Configuration:
  Expiry Duration: 3 months
  Force Rotate: 0
 Autolock Managers: false
 Root Rotation In Progress: false
 Node Address: 192.168.50.4
 Manager Addresses:
  192.168.50.4:2377
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 06b9cb35161009dcb7123345749fef02f7cea8e0
runc version: 3f2f8b84a77f73d38244dd690525642a72156c64
init version: 949e6fa
Security Options:
 apparmor
 seccomp
  Profile: default
Kernel Version: 4.4.0-31-generic
Operating System: Ubuntu 16.04.2 LTS
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 2.915GiB
Name: vagrant
ID: QCSK:UDNT:AMC2:P6TV:M4LV:UZQE:7Z2W:TEHN:VJ75:YQTG:QIBD:TYJC
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): true
 File Descriptors: 85
 Goroutines: 180
 System Time: 2017-09-28T14:10:12.894604072Z
 EventsListeners: 1
Username: cezarsa
Registry: https://index.docker.io/v1/
Experimental: false
Insecure Registries:
 127.0.0.1:5000
Live Restore Enabled: false

Additional environment details (AWS, VirtualBox, physical, etc.):

Virtualbox

[thaJeztah]

Looks to be introduced in docker-archive/docker-ce@30f1b65#diff-27c72b124060b3fc44b4c73d7023fca7R72 (#34188), not sure if this was a deliberate change; Let me open a PR for discussion

[thaJeztah]

Opened #35022