tests.cfg: Display default_install variant in test name

This type of installation is commonly used, remove the '@'
before it.

Also update the comment for it.
Signed-off-by: NTom Zhou <zhouqt@gmail.com>

tests/cfg/unattended_install.cfg

@@ -41,11 +41,12 @@
             image_aio = native
         - aio_threads:
             image_aio = threads
-    # Add some special type install
+    # Add some special types of installation
     variants:
-        - @default_install:
-        #install for performance
+        - default_install:
+            # Installation without any special configuration
         - perf:
+            # configuration for performance test
         - multi_disk_install:
             no ide
             only unattended_install..cdrom