Improve applying condition for SALT_MODELS_SYSTEM_TAG

SALT_MODELS_SYSTEM_TAG was applied only if SALT_MODELS_SYSTEM_COMMIT
is not empty.

Change applying order:
1. SALT_MODELS_SYSTEM_COMMIT, if set
2. else SALT_MODELS_SYSTEM_REF_CHANGE, if set
3. else SALT_MODELS_SYSTEM_TAG

Change-Id: I136abee7d2a76c97ac696d50877abe686449cd5c
diff --git a/tcp_tests/templates/shared-salt.yaml b/tcp_tests/templates/shared-salt.yaml
index b71da48..9084609 100644
--- a/tcp_tests/templates/shared-salt.yaml
+++ b/tcp_tests/templates/shared-salt.yaml
@@ -238,23 +238,24 @@
     {%- elif SALT_MODELS_COMMIT != 'master' %}
     git checkout {{ SALT_MODELS_COMMIT }};
     {%- endif %}
+
     {%- if SALT_MODELS_SYSTEM_COMMIT != '' %}
     pushd classes/system/;
     git checkout {{ SALT_MODELS_SYSTEM_COMMIT }};
     popd;
-    {%- if SALT_MODELS_SYSTEM_TAG != '' %}
-    pushd classes/system/;
-    git fetch --all --tags --prune
-    git checkout tags/{{ SALT_MODELS_SYSTEM_TAG }};
-    popd;
-    {%- endif %}
     {%- elif SALT_MODELS_SYSTEM_REF_CHANGE != '' %}
     pushd classes/system/ && \
     {%- for item in SALT_MODELS_SYSTEM_REF_CHANGE.split(" ") %}
     git fetch {{ SALT_MODELS_SYSTEM_REPOSITORY }} {{ item }} && git cherry-pick FETCH_HEAD;
     {%- endfor %}
     popd;
+    {%- elif SALT_MODELS_SYSTEM_TAG != '' %}
+    pushd classes/system/;
+    git fetch --all --tags --prune
+    git checkout tags/{{ SALT_MODELS_SYSTEM_TAG }};
+    popd;
     {%- endif %}
+
     popd;
     mkdir -p /srv/salt/reclass/classes/service;
     mkdir -p /srv/salt/reclass/nodes/_generated/;
@@ -354,11 +355,10 @@
       echo "SALT_MODELS_SYSTEM_REPOSITORY={{ SALT_MODELS_SYSTEM_REPOSITORY }}"
       {%- if SALT_MODELS_SYSTEM_COMMIT != '' %}
         echo "SALT_MODELS_SYSTEM_COMMIT={{ SALT_MODELS_SYSTEM_COMMIT }}"
-      {%- if SALT_MODELS_SYSTEM_TAG != '' %}
-        echo "SALT_MODELS_SYSTEM_TAG={{ SALT_MODELS_SYSTEM_TAG }}"
-      {%- endif %}
       {%- elif SALT_MODELS_SYSTEM_REF_CHANGE != '' %}
         echo "SALT_MODELS_SYSTEM_REF_CHANGE={{ SALT_MODELS_SYSTEM_REF_CHANGE }}"
+      {%- elif SALT_MODELS_SYSTEM_TAG != '' %}
+        echo "SALT_MODELS_SYSTEM_TAG={{ SALT_MODELS_SYSTEM_TAG }}"
       {%- endif %}
       echo "======================================="
   node_name: {{ HOSTNAME_CFG01 }}
@@ -453,18 +453,17 @@
     pushd /srv/salt/reclass/classes/system/
     git checkout {{ SALT_MODELS_SYSTEM_COMMIT }};
     popd;
-    {%- if SALT_MODELS_SYSTEM_TAG != '' %}
-    pushd /srv/salt/reclass/classes/system/
-    git fetch --all --tags --prune
-    git checkout tags/{{ SALT_MODELS_SYSTEM_TAG }};
-    popd;
-    {%- endif %}
     {%- elif SALT_MODELS_SYSTEM_REF_CHANGE != '' %}
     pushd /srv/salt/reclass/classes/system/ && \
     {%- for item in SALT_MODELS_SYSTEM_REF_CHANGE.split(" ") %}
     git fetch {{ SALT_MODELS_SYSTEM_REPOSITORY }} {{ item }} && git cherry-pick FETCH_HEAD;
     {%- endfor %}
     popd;
+    {%- elif SALT_MODELS_SYSTEM_TAG != '' %}
+    pushd /srv/salt/reclass/classes/system/
+    git fetch --all --tags --prune
+    git checkout tags/{{ SALT_MODELS_SYSTEM_TAG }};
+    popd;
     {%- endif %}
 
     {%- if IS_CONTRAIL_LAB %}