samples/net/dns: Improve network configuration

- Reorganize some CONF variables and add the CONFIG_NET_SAMPLES_xxx
  variables to the DNS sample application.
- Remove all the numeric IP addresses
- Use the CONFIG_NET_SAMPLES_xxx variables to define the IP addresses

Change-Id: Ib294d338f163f03ea1b47143b489d916578c532d
Signed-off-by: Flavio Santes <flavio.santes@intel.com>
5 files changed