Add support of DNS names for a server to query (dns_check)

[anesmashnyi-godaddy]

We periodically switch CNAME's for nameservers.

For now, dns check only allows to use IP addresses
https://github.com/DataDog/integrations-core/blob/master/dns_check/datadog_checks/dns_check/data/conf.yaml.example

    ## @param nameserver - string - optional
    ## IP address of the name server to query.
    #
    # nameserver: <NAMESERVER>

Could you please add DNS name support for nameserver parameter ?