loadtester: add concord test support

[tebo-ernstberger]

Hi,

Here's a PR for Concord support in Flagger's loadtester.

Fix: #477

Example query

{
    "metadata": {
        "type": "concord",
        "server": "https://concord.prod.foo.com/",
        "org": "my-concord-repo-org",
        "project": ""my-concord-project",
        "repo": "my-concord-repo",
        "entrypoint": ""my-concord-entrypoint",
        "apiKeyPath": "/tmp/concord-api-key",
        "endpoint": "https://canary-endpoint/",
        "pollInterval": "5",
        "pollTimeout": "60"
    }
}

Example logs

{"level":"info","ts":"2020-03-17T13:54:07.723-0700","caller":"loadtester/concord.go:173","msg":"calling endpoint https://concord.prod.foo.com/api/v1/process","canary":""}
{"level":"info","ts":"2020-03-17T13:54:07.874-0700","caller":"loadtester/concord.go:208","msg":"created process id [4e7f4245-85aa-4c33-a56a-d4df5b63d3fe] OK status is true","canary":""}
{"level":"info","ts":"2020-03-17T13:54:12.875-0700","caller":"loadtester/concord.go:173","msg":"calling endpoint https://concord.prod.foo.com/api/v1/process/4e7f4245-85aa-4c33-a56a-d4df5b63d3fe","canary":""}
{"level":"info","ts":"2020-03-17T13:54:13.010-0700","caller":"loadtester/concord.go:227","msg":"process id [4e7f4245-85aa-4c33-a56a-d4df5b63d3fe] current status is STARTING","canary":""}
{"level":"info","ts":"2020-03-17T13:54:17.874-0700","caller":"loadtester/concord.go:173","msg":"calling endpoint https://concord.prod.foo.com/api/v1/process/4e7f4245-85aa-4c33-a56a-d4df5b63d3fe","canary":""}
{"level":"info","ts":"2020-03-17T13:54:17.985-0700","caller":"loadtester/concord.go:227","msg":"process id [4e7f4245-85aa-4c33-a56a-d4df5b63d3fe] current status is STARTING","canary":""}
{"level":"info","ts":"2020-03-17T13:54:22.874-0700","caller":"loadtester/concord.go:173","msg":"calling endpoint https://concord.prod.foo.com/api/v1/process/4e7f4245-85aa-4c33-a56a-d4df5b63d3fe","canary":""}
{"level":"info","ts":"2020-03-17T13:54:23.061-0700","caller":"loadtester/concord.go:227","msg":"process id [4e7f4245-85aa-4c33-a56a-d4df5b63d3fe] current status is STARTING","canary":""}
{"level":"info","ts":"2020-03-17T13:54:27.878-0700","caller":"loadtester/concord.go:173","msg":"calling endpoint https://concord.prod.foo.com/api/v1/process/4e7f4245-85aa-4c33-a56a-d4df5b63d3fe","canary":""}
{"level":"info","ts":"2020-03-17T13:54:27.986-0700","caller":"loadtester/concord.go:227","msg":"process id [4e7f4245-85aa-4c33-a56a-d4df5b63d3fe] current status is RUNNING","canary":""}
{"level":"info","ts":"2020-03-17T13:54:32.876-0700","caller":"loadtester/concord.go:173","msg":"calling endpoint https://concord.prod.foo.com/api/v1/process/4e7f4245-85aa-4c33-a56a-d4df5b63d3fe","canary":""}
{"level":"info","ts":"2020-03-17T13:54:32.992-0700","caller":"loadtester/concord.go:227","msg":"process id [4e7f4245-85aa-4c33-a56a-d4df5b63d3fe] current status is FINISHED","canary":""}

Regarding Authentication
This implementation requires to have a Concord API key present in a file directly on the container. This can be done via mounting a k8s secret in the Loadtester's Deployment.
I went with a file over an environment variable since it's arguably more secured. Happy to switch if you think necessary.

Let me know what you think so far,

Best,

[stefanprodan]

@ernst01 thanks for the PR, can you please document the webhook here https://github.com/weaveworks/flagger/blob/master/docs/gitbook/usage/webhooks.md#integration-testing

Thanks

[stefanprodan]

LGTM

Thanks @ernst01 🏅