Validate list_keypair attribute of Nova V2/V3 APIs

This patch adds the JSON Schema for response of Nova V2 & V3
list_keypair APIs and validate the response with added JSON Schema to
block the backward incompatibility change in the future.

The response body of V2 & V3 list_keypairs APIs is same and given below:

{
    "keypairs": [
        {
            "keypair": {
                "public_key": "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQD
                              nPe3MJhEwcQNGZpYRaGYTwY9D9L3pr1c+eqJIP99
                              mtrUuqTEp3/YPiAzpL0Dem/32WWPOV4DEc27NBMx
                              rpOi9JJBssDq+6UdK5hMIwrZ/6dZt+CPz+76Lwxn
                              R1vilVFocis+OC9ddntazV1wrAWGyYL9a83vwcuT
                              V01FJzSNlYV4hni2vxcgQPkbgt+ldYoTmBS9X3W5
                              Qy/NrzuG53zHjkYjmz3tjiadZwgffDvjT3SBYWU3
                              kBGy6gxIs5f/wIvZUSUXGorOJNUw6FubMj4sLfE4
                              cbHS6W72xs6bmEhqxyjSLQK5fuHE1MgCCDPTd0vq
                              XdbLnCZ6+l0I8lpDhd Generated by Nova\n",
                "name": "test_key",
                "fingerprint": "52:53:dd:89:6a:73:c6:18:87:4b:1e:c1:4c
                               :b8:c3:c9"
            }
        },
        {
            "keypair": {
                "public_key": "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC
                              9E3tf6msdTyVcytQrs882e4aX3Zhkp1ELIAowspL
                              OykUbzq21+0JALCxMkUxXig2GCF3jomfWez1frhG
                              o+Qj/ZgdwJG44k1ZzLswPZn5k5SMQzROTsnzDLXX
                              f95q6fWZLlfAbF7MA/wd/d/7oBmmrTfaiV27Xq9B8
                              x/TlWldmMd1Cst6SEYkeVhaynlxaFnrW9sr1nKGec
                              XSOtVagtUFP4XPxsN1XbAwxZX+ZeWMTYUgliD4+Sm
                              QevNuWfpHXPSTZ947GMxMythCIPjmZ34W4wq5xVI9
                              o1MCgL7s2nLUDqYm/DesB+m9MI/ylCpLVqwFDdDHg
                              rI9x+fRYInsHbUuF Generated by Nova\n",
                "name": "test_key2",
                "fingerprint": "ea:51:b2:56:85:b3:fb:43:6f:b7:a5:54:b7:
                               bd:ab:6d"
            }
        }
    ]
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Ie500239af2e862c14d49daac0e8018b24c92982b
Merge "Validate list_keypair attribute of Nova V2/V3 APIs"