fix: delete manifest before blobs by default

Currently the conformance tests default to deleting manifests after
blobs, which leaves the manifest dangling. Furthermore, the same blob may be
referenced in many manifests.

AFAIK, no client does this.

So changing default behavior.

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

conformance/README.md

@@ -31,7 +31,7 @@ export OCI_TEST_CONTENT_MANAGEMENT=1
 # Extra settings
 export OCI_HIDE_SKIPPED_WORKFLOWS=0
 export OCI_DEBUG=0
-export OCI_DELETE_MANIFEST_BEFORE_BLOBS=0
+export OCI_DELETE_MANIFEST_BEFORE_BLOBS=0 (defaults to OCI_DELETE_MANIFEST_BEFORE_BLOBS=1 if not set)
 ```
 
 Lastly, run the tests:

conformance/setup.go

@@ -310,7 +310,7 @@ func init() {
 	runContentDiscoverySetup = true
 	runContentManagementSetup = true
 	skipEmptyLayerTest = false
-	deleteManifestBeforeBlobs = false
+	deleteManifestBeforeBlobs = true
 
 	if os.Getenv(envVarTagName) != "" &&
 		os.Getenv(envVarManifestDigest) != "" &&
@@ -323,7 +323,9 @@ func init() {
 	}
 
 	skipEmptyLayerTest, _ = strconv.ParseBool(os.Getenv(envVarPushEmptyLayer))
-	deleteManifestBeforeBlobs, _ = strconv.ParseBool(os.Getenv(envVarDeleteManifestBeforeBlobs))
+	if v, ok := os.LookupEnv(envVarDeleteManifestBeforeBlobs); ok {
+		deleteManifestBeforeBlobs, _ = strconv.ParseBool(v)
+	}
 	automaticCrossmountVal := ""
 	automaticCrossmountVal, runAutomaticCrossmountTest = os.LookupEnv(envVarAutomaticCrossmount)
 	automaticCrossmountEnabled, _ = strconv.ParseBool(automaticCrossmountVal)