Turn verify_host_resolvable() into a wrapper around ipapython.dnsutil
    
    The code was duplicate and less generic anyway.
    As a side-effect I had to re-wrap dns.exception.DNSException into a
    PublicError so it can be displayed to the user.
    
    DNSError is now a super class for other DNS-related errors. Errors from
    DNS resolver are re-raised as DNSResolverError.
    
    https://fedorahosted.org/freeipa/ticket/5710
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>