plugins: add a release only flag to Discover

Since we now support loading pre-releases, we also want Packer to be
able to ignore them by user demand, so we put in place the
infrastructure to modulate this.

command/init_test.go

@@ -263,7 +263,7 @@ func TestInitCommand_Run(t *testing.T) {
 				Meta: tt.Meta,
 			}
 
-			if err := c.CoreConfig.Components.PluginConfig.Discover(); err != nil {
+			if err := c.CoreConfig.Components.PluginConfig.Discover(false); err != nil {
 				t.Fatalf("Failed to discover plugins: %s", err)
 			}