add secrets api and inventary table update

[bybai]

For https://github.ibm.com/xcat2/task_management/issues/145

Description:

1. add updating and adding xCAT table entry in xcat-inventory
2. add secrets post API

UT:

1. /inventory/secrets	POST	create a new secret object

   curl -X POST "http://10.4.41.7:5000/api/v2/security/secrets" -H  "accept: application/json" -H  "Content-Type: application/json" -d "{    \"kind\": \"test\",    \"spec\": {      \"username\": \"xc\",      \"password\": \"cluster\"    }}"
   {
   "message": "new user is created.",
   "id": "test_xc"
   }

2. /inventory/secrets/{id}	POST	modify a specified type of secrets object

   
   curl -X POST "http://10.4.41.7:5000/api/v2/security/secrets/test_xcx" -H  "accept: application/json" -H  "Content-Type: application/json" -d "{    \"kind\": \"test\",    \"spec\": {      \"username\": \"x\",      \"password\": \"aaabbbbb\"    }}"
   {
   "message": "It is not the same user."
   }

curl -X POST "http://10.4.41.7:5000/api/v2/security/secrets/test_x" -H "accept: application/json" -H "Content-Type: application/json" -d "{ \"kind\": \"test\", \"spec\": { \"username\": \"x\", \"password\": \"aaxxxkkkkn\" }}" { "message": "Update user test_x successfully." }