PRD-1290 / do not use ping to verify DNS availability

Do not use ping in the DNS availability test. Instead, resolve reverse DNS records for a global IP address (8.8.8.8) and a local IP address (IP address of the Fuel master, parsed out of /etc/resolv.conf).

NB: An alternative solution was discussed to split the DNS availability test into two tests: 1) ping test to confirm access to the Internet (a VM provisioned with a floating IP would be needed to pass such test in configurations where compute node itself has no external routes); 2) DNS test to resolve 8.8.8.8. Resolution of a local IP address would not be a part of such test.

This change is not intended for master, yet, and should not be merged until the above solution is finalized.