pkg/utils: Re-arrange and rename the default and fallback values

The terms 'default' and 'fallback' are used to mean very specific
things in this context.

The 'default' values are those that are used when the 'create',
'enter' and 'run' commands were used without any other option.  These
values are picked to match the host operating system.

However, if there's no supported Toolbox image matching the host
operating system, and no options were provided to the 'create', 'enter'
and 'run' commands, then the 'fallback' values are used as a last
resort.

Consistently using this terminology leads to a clear mental model and
makes the code easier to read.

This rough arrangement of the code was already being used for the
'release', and has now been been extended to the 'container name
prefix' and 'distro'.  The suffix for the 'fallback' values was
simplified to 'Fallback', instead of 'DefaultFallback'.

containers#937

src/pkg/utils/utils.go

@@ -50,8 +50,10 @@ type Distro struct {
 }
 
 const (
-	idTruncLength          = 12
-	releaseDefaultFallback = "34"
+	containerNamePrefixFallback = "fedora-toolbox"
+	distroFallback              = "fedora"
+	idTruncLength               = 12
+	releaseFallback             = "34"
 )
 
 const (
@@ -61,9 +63,9 @@ const (
 )
 
 var (
-	containerNamePrefixDefault = "fedora-toolbox"
+	containerNamePrefixDefault string
 
-	distroDefault = "fedora"
+	distroDefault string
 
 	preservedEnvironmentVariables = []string{
 		"COLORTERM",
@@ -118,7 +120,9 @@ var (
 )
 
 func init() {
-	releaseDefault = releaseDefaultFallback
+	containerNamePrefixDefault = containerNamePrefixFallback
+	distroDefault = distroFallback
+	releaseDefault = releaseFallback
 
 	hostID, err := GetHostID()
 	if err == nil {
@@ -247,7 +251,7 @@ func getDefaultImageForDistro(distro, release string) string {
 	}
 
 	if _, supportedDistro := supportedDistros[distro]; !supportedDistro {
-		distro = "fedora"
+		distro = distroFallback
 	}
 
 	distroObj, supportedDistro := supportedDistros[distro]
@@ -614,7 +618,7 @@ func ParseRelease(distro, release string) (string, error) {
 	}
 
 	if _, supportedDistro := supportedDistros[distro]; !supportedDistro {
-		distro = "fedora"
+		distro = distroFallback
 	}
 
 	distroObj, supportedDistro := supportedDistros[distro]