Describe the bug
Cannot PUT a new monitor with a specified ID for a non-existant monitor:

PUT _opendistro/_alerting/monitors/flibble
{
  "type": "monitor",
  "name": "flibble",
  "enabled": true,
  "schedule": {
    "period": {
      "interval": 10,
      "unit": "MINUTES"
    }
...

Results in:

{
  "Message" : "Monitor with flibble is not found"
} {
  "_index" :

How to reproduce?

Run go test ./... at the root in feat/es7 branch

You can easily see that a lot of test cases get failed, we need to fix it and make sure that all the test cases work as expected.

Additional Context

We recently upgraded the elastic search version from 6.x to 7.x which has some breaking changes, probably that might be the reason for some failed test cases.