Use master flag for disabling BWC tests

This commit skips the the BWC tests if the master BWC flag
bwc_tests_enabled in core is set to false.

Relates elastic/x-pack-elasticsearch#1725

Original commit: elastic/x-pack-elasticsearch@7b924066a9

qa/rolling-upgrade/build.gradle

@@ -209,14 +209,18 @@ subprojects {
       dependsOn = [upgradedClusterTest]
     }
 
-    bwcTest.dependsOn(versionBwcTest)
+    if (project.bwc_tests_enabled) {
+      bwcTest.dependsOn(versionBwcTest)
+    }
   }
 
   test.enabled = false // no unit tests for rolling upgrades, only the rest integration test
 
   // basic integ tests includes testing bwc against the most recent version
   task integTest {
-    dependsOn = ["v${wireCompatVersions[-1]}#bwcTest"]
+    if (project.bwc_tests_enabled) {
+      dependsOn = ["v${wireCompatVersions[-1]}#bwcTest"]
+    }
   }
   check.dependsOn(integTest)