merge Jenkins node labels

Looking at Jenkins node usage, we're seeing that rspec / normal docker executors can be sitting idle while waiting for a selenium executor. There shouldn't be a difference between their configurations, so let's see how it affects the queue times if we make all stages use the same type of node.

refs DE-276

Change-Id: Ic8f9ffe8d148dc9119d1e456e689b46973a1fb3b
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/248114
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Kyle Rosenbaum <krosenbaum@instructure.com>
QA-Review: Aaron Ogata <aogata@instructure.com>
Product-Review: Aaron Ogata <aogata@instructure.com>

build/new-jenkins/groovy/distribution.groovy

@@ -41,7 +41,7 @@ def appendStagesAsBuildNodes(nodes,
     def stage_name = "$stage_name_prefix ${(index + 1).toString().padLeft(2, '0')}"
     def timeStart = new Date()
     nodes[stage_name] = {
-      protectedNode("canvas-$test_label-docker") {
+      protectedNode("canvas-docker") {
         echo "Running on node ${env.NODE_NAME}"
         def duration = TimeCategory.minus(new Date(), timeStart).toMilliseconds()
         // make sure to unstash