[W.I.P.] Use yaml.safe_load rather than yaml.load

[hartwork]

Mostly aiming at:

• addressing security warning that tools might show
• fixing the test suite for operating systems that path yaml.load to be disallowed

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "yaml-safe-load" [email protected]:hartwork/docker-compose.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[hartwork]

Closing as a duplicate, found #6623 just now