Validate get limits attributes of Nova V2 API

Now most attributes of Nova v2/v3 APIs are not checked in Tempest,
and this patch adds some tests which check these attributes to block
the backward incompatibility change in the future.

This patch adds the checks of "get limits" API responses.

The response body of v2 API is the following:
  {
    "limits": {
      "absolute": {
        "maxTotalRAMSize": 51200,
        "totalCoresUsed": 0,
        "maxTotalInstances": 10,
        "maxTotalFloatingIps": 10,
        "totalSecurityGroupsUsed": 0,
        "maxTotalCores": 20,
        "totalFloatingIpsUsed": 0,
        "maxSecurityGroups": 10,
        "maxServerMeta": 128,
        "maxPersonality": 5,
        "maxImageMeta": 128,
        "maxPersonalitySize": 10240,
        "maxSecurityGroupRules": 20,
        "maxTotalKeypairs": 100,
        "totalRAMUsed": 0,
        "totalInstancesUsed": 0
      },
      "rate": []
    }
  }

Partially implements blueprint nova-api-attribute-test

Change-Id: I503db2986ddcafb899f16d1547fd4857d355f2ed
Merge "Validate get limits attributes of Nova V2 API"