provisioners/ansible: try to improve unit tests

I still cannot explain the cause of these random errors in this unit test,
but it is anyway safe and suitable to update the test code as following:
- use stricter regular expression matching (-l is included in --limit)
- array lengths substraction instead of array contents substraction
Merge pull request #3632 from gildegoma/ansible-random-errors

provisioners/ansible: try to improve unit tests