Add vagrant boxes caching to CI

We're hitting 429 responses when running lots of jobs, presumably
because we are downloading lots of boxes again and again.
This should cache them instead.

.gitlab-ci/molecule.yml

@@ -29,6 +29,10 @@
     when: always
     paths:
     - molecule_logs/
+  cache:
+    key: vagrant_boxes
+    paths:
+    - ~/vagrant.d/boxes/
 
 # CI template for periodic CI jobs
 # Enabled when PERIODIC_CI_ENABLED var is set

.gitlab-ci/vagrant.yml

@@ -27,6 +27,10 @@
     - ./tests/scripts/vagrant_clean.sh
   script:
     - ./tests/scripts/testcases_run.sh
+  cache:
+    key: vagrant_boxes
+    paths:
+    - ~/vagrant.d/boxes/
 
 vagrant_ubuntu20-calico-dual-stack:
   stage: deploy-extended