Alerting: Introduces /api/v1/ngalert/alertmanagers to expose discovered and dropped Alertmanager(s) (#37632)

* Alerting: Expose discovered and dropped Alertmanagers

Exposes the API for discovered and dropped Alertmanagers.

* make admin config poll interval configurable

* update after rebase

* wordsmith

* More wordsmithing

* change name of the config

* settings package too

pkg/tests/api/alerting/testing.go

@@ -56,10 +56,10 @@ func postRequest(t *testing.T, url string, body string, expStatusCode int) *http
 	buf := bytes.NewReader([]byte(body))
 	// nolint:gosec
 	resp, err := http.Post(url, "application/json", buf)
+	require.NoError(t, err)
 	t.Cleanup(func() {
 		require.NoError(t, resp.Body.Close())
 	})
-	require.NoError(t, err)
 	if expStatusCode != resp.StatusCode {
 		b, err := ioutil.ReadAll(resp.Body)
 		require.NoError(t, err)