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
guests/debian: fix tests
guests/debian: more tests for hostname
providers/virtualbox: don't do transient by default
Use 'tee' to write to /etc/exports
Fix up the Linux host too
providers/docker: vagrant_machine setting works proprely
providers/hyperv: lower priority for Windows
providers/virtualbox: higher priority than default
website/docs: document global-status upgrade notes
core: Environment#cwd should expand path to full path
core: MachineIndex#valid? checks state and ID again
providers/docker: smarter about setting ID to nil for machine
providers/docker: less hack way to clear ID if not created
core: if state ID is NOT_CREATED_ID, destroy machine state
core: MachineIndex valid test is better
providers/*: use special NOT_CREATED_ID if not created
providers/docker: smarter destroy, does not require host machine
providers/docker: don't reload if not created