compose parser fails 3.4+ schema for `volume` with `external`, `driver`, `driver_opts`, `labels`

Description

The compose parser in /cli/compose/loader/ does not follow the compose 3.4+ schema rules as they apply to volumes that are external and also declare driver, driver_opts, labels. Instead, the code only follows the rules that apply for schemas 3.0-3.3.

Errant code has been isolated and trivial fix is ready.

Setup

• https://github.com/docker/cli 18.06, 18.09, or current master in 19.03
• go code that loads/parses a compose file using high-level functions like loader.Load() or the low-level errant function loader.LoadVolumes().
• parse a docker-compose.yml that is version=3.4, has a top-level volumes: that declares a volume with both external:true and driver, driver_opts, or labels

Repro

1. Setup as above
2. Parse a docker-compose similar to the following

   version: '3.4'
   volumes:
     myVol:
       external: true
       driver: 'rexray/ebs:0.11.4'
   services:
     web:
       image: ubuntu
       volumes:
         - myVol:/mnt/one
   

Result

Error. conflicting parameters "external" and driver specified for volume myVol

Expected

No error. And to have a fully parsed Volume section.

Notes

The errant code is below. You can see the old 3.0-3.3 rules of mutual exclusion are being applied to all schema versions. The fix is to only apply them to versions 3.0-3.3 as per Docker documentation https://docs.docker.com/compose/compose-file/#external

https://github.com/docker/cli/blob/52714e413cd1f58d5f3734191ad23a4701a9d8e3/cli/compose/loader/loader.go#L579-L595

This is related to old work that didn't implement the full set of rules for 3.4+. Issues https://github.com/docker/cli/issues/274 and https://github.com/docker/cli/issues/608

Trivial fix. I have a commit that I'm using which was based on the 18.09.5 release. Also trivial to apply to the 18.06 and 19.03 releases. Here is the fix https://github.com/diablodale/cli/commit/1d4c6d8e492c0598c432314be76e5f59d4c60386

Happy to make three separate PRs with fixes and test cases for the three branches 18.06, 18.09, and 19.03.

Start in cli/compose/loader/loader.go around lines 579-595 and reproduce the version 3.4 volume case using loader.Load() or loader.LoadVolumes(). Add coverage for external volumes with driver, driver_opts, and labels; done means the compose file parses without a conflict error and produces a fully parsed Volume section.