Aligned Nameservers property with Redfish Spec

- As per the below mentioned proposal,i have added a new
  property StaticNameServers & also modified the definition
  of the existing Nameservers property

https://lists.ozlabs.org/pipermail/openbmc/2019-September/018399.html

- As mentioned in the proposal:

[x] Nameservers - will now be only a readonly Property, and it
                  contains all the DNS servers(Static & Dynamic)
	          configured on the interface.
- This should support NotAllowed Error, so that if a user tries
  to patch this property,it returns operation NotAllowed as status.

[x] StaticNameServers - will be a writable property,which user can
			patch with a list of IP address(s).
- This should support InvalidArgument Error, so that if user tries
  to patch with some junk data, we can return InvalidArgument when
  the backend validation fails.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: I3d85463c7e37daf9f98f1f35f4efeaeb52714291
diff --git a/xyz/openbmc_project/Network/EthernetInterface.interface.yaml b/xyz/openbmc_project/Network/EthernetInterface.interface.yaml
index e7fe048..56285ee 100644
--- a/xyz/openbmc_project/Network/EthernetInterface.interface.yaml
+++ b/xyz/openbmc_project/Network/EthernetInterface.interface.yaml
@@ -29,10 +29,19 @@
     - name: Nameservers
       type: array[string]
       description: >
-          DNS servers on the interface.
+          DNS servers that are currently in use on this interface.
       errors:
-          - xyz.openbmc_project.Common.Error.InvalidArgument
           - xyz::openbmc_project.Common.Error.NotAllowed
+    - name: StaticNameServers
+      type: array[string]
+      description: >
+          A statically defined set of DNS server IP addresses to be used when
+          DHCP provisioning is not enabled for name server configuration. As
+          an implementation option this may also be used in addition to DHCP
+          provided addresses, or in cases where the DHCP server provides no DNS
+          assigments.
+      errors:
+         - xyz.openbmc_project.Common.Error.InvalidArgument
     - name: NTPServers
       type: array[string]
       description: >