Vagrant will verify that the current Ansible version does support the
requested compatibility mode (only applicable if not "auto", of course).
As mentioned in the documentation, there is no sanity checks between
`version` option and `compatibility_mode` option.
With this change, the host-based provisioner is also improved to
execute only once the "ansible" command (and store the gathered
information for multiple usages like version requirement and
compatibility checks). On the other hand, the guest-based provisioner
can still potentially execute "ansible" twice (once in the
AnsibleInstalled cap, and via "gather_ansible_version" function via
Base::set_compatibility_mode).
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