defaultCredentialStore: make this a function

In the next patch, we'll use this to implement some logic about which
password backend to use.

Signed-off-by: Tycho Andersen <tycho@docker.com>

cli/config/credentials/default_store.go

@@ -7,13 +7,15 @@ import (
 // DetectDefaultStore return the default credentials store for the platform if
 // the store executable is available.
 func DetectDefaultStore(store string) string {
+	platformDefault := defaultCredentialsStore()
+
 	// user defined or no default for platform
-	if store != "" || defaultCredentialsStore == "" {
+	if store != "" || platformDefault == "" {
 		return store
 	}
 
-	if _, err := exec.LookPath(remoteCredentialsPrefix + defaultCredentialsStore); err == nil {
+	if _, err := exec.LookPath(remoteCredentialsPrefix + platformDefault); err == nil {
-		return defaultCredentialsStore
+		return platformDefault
 	}
 	return ""
 }

cli/config/credentials/default_store_darwin.go

@@ -1,3 +1,5 @@
 package credentials
 
-const defaultCredentialsStore = "osxkeychain"
+func defaultCredentialsStore() string {
+	return "osxkeychain"
+}

cli/config/credentials/default_store_linux.go

@@ -1,3 +1,5 @@
 package credentials
 
-const defaultCredentialsStore = "secretservice"
+func defaultCredentialsStore() string {
+	return "secretservice"
+}

cli/config/credentials/default_store_windows.go

@@ -1,3 +1,5 @@
 package credentials
 
-const defaultCredentialsStore = "wincred"
+func defaultCredentialsStore() string {
+	return "wincred"
+}