add new dcm e2e tests

Add DCM e2e tests focused on user experience.

Here are some requisites and thoughts kept in mind while designing and
coding the tests:

* Document what the test should do and its expectations.
* Make it as much focused on user experience as possible: create the
  scenarios and validate the result checking the behavior in the same
  way the user would observe it.
* Make it more declarative: create the pre-requisite for the test inside
  the test itself, instead of relying on pre-configured requisites.
* Make it simple to follow: try to abstract complexities into higher
  level calls, making the test itself smaller, but also comprehensive.

https://redhat.atlassian.net/browse/NE-2529

Author: jcmoraisjr

test/extended/router/config_manager.go

@@ -31,6 +31,9 @@ import (
 
 const timeoutSeconds = 3 * 60
 
+const routerSelectorKey = "select"
+const routerSelectorValue = "haproxy-cfgmgr"
+
 var _ = g.Describe("[sig-network][Feature:Router][apigroup:route.openshift.io]", func() {
 	defer g.GinkgoRecover()
 	var (
@@ -846,7 +849,7 @@ func createRoute(oc *exutil.CLI, routeType routeType, routeName, serviceName, ho
 	if err != nil {
 		return err
 	}
-	return oc.AsAdmin().Run("label").Args("route", routeName, "select=haproxy-cfgmgr").Execute()
+	return oc.AsAdmin().Run("label").Args("route", routeName, routerSelectorKey+"="+routerSelectorValue).Execute()
 }
 
 func readURL(ns, execPodName, host, abspath, ipaddr string) (string, error) {