Use image ref instead of tags so we don't need to have different image tags for master/develop. (#2834)

.yamato/csharp-tests.yml

   name: Test Mac EditMode {{ editor.version }}
   agent:
     type: Unity::VM::osx
-    image: ml-agents/ml-agents-bokken-mac:stable
+    image: ml-agents/ml-agents-bokken-mac:v0.1.2-440635
     flavor: i1.small
   variables:
     UNITY_VERSION: {{ editor.version }}

.yamato/standalone-build-test.yml

   name: Test Mac Standalone {{ editor.version }}
   agent:
     type: Unity::VM::osx
-    image: ml-agents/ml-agents-bokken-mac:stable
+    image: ml-agents/ml-agents-bokken-mac:v0.1.2-440635
     flavor: i1.small
   variables:
     UNITY_VERSION: {{ editor.version }}