fix: set image version before switching to the configuration file (el…

…astic#565)

We still need to download the proper config file from the PR, but we do not
have access to the commit SHA at this moment, unless other moddifications
are in place, like reading from Jenkins

e2e/_suites/metricbeat/metricbeat_test.go

@@ -246,14 +246,14 @@ func (mts *MetricbeatTestSuite) installedAndConfiguredForVariantModule(serviceVa
 }
 
 func (mts *MetricbeatTestSuite) installedUsingConfiguration(configuration string) error {
-	if strings.HasPrefix(metricbeatVersion, "pr-") {
-		metricbeatVersion = metricbeatVersionBase
-	}
-
 	// at this point we have everything to define the index name
 	mts.Version = metricbeatVersion
 	mts.setIndexName()
 
+	if strings.HasPrefix(metricbeatVersion, "pr-") {
+		metricbeatVersion = metricbeatVersionBase
+	}
+
 	// use master branch for snapshots
 	tag := "v" + metricbeatVersion
 	if strings.Contains(metricbeatVersion, "SNAPSHOT") {