Vagrantfile: do not use dns 8.8.8.8 if unreachable

[giner]

Remove 8.8.8.8 from /etc/resolv.conf and /etc/resolvconf/resolv.conf.d/head if not reachable. External DNS is often blocked in private networks. In order avoid DNS timeouts we check if 8.8.8.8 is reachable when provisioning.

[cfdreddbot]

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[cppforlife]

@giner I think it's better to introduce a new env variable that allows to explicitly set DNS entries (or recomment people to use vagrant-dns). Also most vagrantfile changes should be done in the packaged vagrantfile instead of the example Vagrantfile: https://github.com/cloudfoundry/bosh-lite/blob/master/packer/templates/vagrant-local.tpl

[cppforlife]

Closing as per comment above.

[giner]

Normally DNS configuration is done by DHCP of your hypervisor and placing 8.8.8.8 as a primary DNS does not seem right. Therefore I would revert the following commit in this case d984f8c @cppforlife