Add condition for docker pull operation

By default, when docker plugin is used in pipelines
we always pull image as a separate action. Sometimes it may hurt
i.e. when registry is accessible only with username and password.
This w/a will implement a switch that can enable or disable pull. 

Related-PROD: PROD-31909

Change-Id: I611a1328da4996d839096796e6e148f35eb91763
diff --git a/src/com/mirantis/mk/SaltModelTesting.groovy b/src/com/mirantis/mk/SaltModelTesting.groovy
index 8fe116f..1b9b3e8 100644
--- a/src/com/mirantis/mk/SaltModelTesting.groovy
+++ b/src/com/mirantis/mk/SaltModelTesting.groovy
@@ -118,7 +118,11 @@
     }
     def img = docker.image(dockerImageName)
 
-    img.pull()
+    def pull_enabled = config.get('dockerPull', true)
+
+    if ( pull_enabled ) {
+        img.pull()
+    }
 
     try {
         img.inside(dockerOptsFinal) {