(NETDEV-23) Remove domain_name, name_server, search_domain

Without this patch the DNS related types are overly complicated by being
separated out into three distinct types.  This patch addresses the
problem by removing the three distinct types in preparation for a
single, consolidated type that looks like:

network_dns
 * domain (string, namevar)
 * search (array of dns names)
 * servers (array of dns servers)
(NETDEV-23) Add network_dns type

Without this patch there is no type to manage the DNS configuration of a
target device.  This patch addresses the problem by adding the
network_dns type with a namevar of `domain` and array properties of
`search` and `servers`.  The type is ensurable because DNS may not be
configured on the target system.