[Arista] Update Arista 7260 lab example data (#2948)

Description of PR
Summary:
There's not enough data from sonic-mgmt for device facts fetched by platform API.
In Arista testbed we use veos file, the example lab file needs more fields in order to pass the platform API tests.

Approach
What is the motivation for this PR?
Some platform API tests fail because it cannot fetch the values for device data from duthost_vars.
https://github.com/Azure/sonic-mgmt/blob/master/tests/platform_tests/api/test_chassis.py#L61

How did you do it?
Added additional fields for Arista 7260 in lab file.

ansible/lab

@@ -94,11 +94,25 @@ sonic_s6100:
 
 sonic_a7260:
   vars:
+    hwsku: Arista-7260CX3-C64
     iface_speed: 100000
   hosts:
     lab-a7260-01:
       ansible_host: 10.251.0.191
-      hwsku: Arista-7260CX3-D108C8
+      model: DCS-7260CX3-64
+      serial: SSJ12345678
+      base_mac: 76:34:ab:08:cd:a0
+      syseeprom_info:
+        "0x21": "DCS-7260CX3-64"
+        "0x22": "ASY0250504B0"
+        "0x23": "SSJ12345678"
+        "0x24": "7634ab08cda0"
+        "0x25": "2017/06/07 12:36:05"
+        "0x26": "01"
+        "0x27": "03.00"
+        "0x28": "x86_64-arista_7260cx3_64"
+        "0x2d": "Arista Networks"
+        "0x2e": "Aboot-norcal7-7.2.3-pcie2x4-12345678"
 
 sonic_multi_asic:
   vars: