ci: Optimize parallel build

* style: Rename TASK to STEP
* do not pull base image after it was copied from node0 and loaded.
  It caused the current Dockerfile to be out-of-sync with the current image.
* do not pull images after loading the downloaded and up-to-date
  base.tar. It was overwriting the base.tar downloaded and loaded.
* save debian:jessie into base.tar
* use "time" command to help identify long running operation
* rename marker file from "BASE_READY" to "BASE_AVAILABLE"
* build base image only if needed.

circle.yml

@@ -21,7 +21,7 @@ test:
         parallel: true
         timeout: 3000
         environment:
-          TASK: test
+          STEP: test
 
 deployment:
   hub:
@@ -30,4 +30,4 @@ deployment:
       - circleci-matrix:
           parallel: true
           environment:
-            TASK: deploy
+            STEP: deployment