Backend changes for Populating Nameservers(DNS & Static)

- As per the proposed design :
https://lists.ozlabs.org/pipermail/openbmc/2019-September/018399.html

Depends on :
https://gerrit.openbmc-project.xyz/#/c/openbmc/phosphor-dbus-interfaces/+/26060/

The idea of this commit is to

- Support NameServers(Read Only) property to display current
  Nameservers (both DHCP provided& Static) configured on the
  interface.

- Support StaticNameServers(Writable) property by which user
  can set the Nameservers on an interface.

Tested By:

1.Configure a DNS via DHCP Server & Make sure NameServer property
  populates accordingly.
2.With DNS from DHCP existing, set a name server using PATCH on
  StaticNameServer property & make sure both properties populate
  the data as per the proposal.
3.Make sure /etc/resolv.conf is populated with the right content
  in case of DHCP/Static and DHCP & Static (Co-existing)
4.Make sure network configuration file is updated properly when
  user sets a staic name server.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: If10b9aa683d2b50e51780e91323c6d10a5ec3710
8 files changed