labs: make headless install the default

This changeset makes "headless" the default GUI type for VirtualBox VMs
during installation.

Change-Id: I438988f01cb82b7a16c0baa62c93bb5f0f4af0e0

labs/config/provider.virtualbox

@@ -15,4 +15,7 @@
 # VirtualBox VM group
 : ${VM_GROUP:=oslabs}
 
+# VirtualBox VM GUI type
+: ${VM_UI:=headless}
+
 # vim: set ai ts=4 sw=4 et ft=sh:

labs/osbash.sh

@@ -62,6 +62,7 @@ function print_config {
 
         echo "VM access method: $VM_ACCESS"
 
+        # GUI is the VirtualBox default
         echo "GUI type: ${VM_UI:-gui}"
     fi
 
@@ -86,8 +87,7 @@ while getopts :be:g:hnw opt; do
             if [[ "$OPTARG" =~ (headless|gui|sdl) ]]; then
                 VM_UI=$OPTARG
             else
-                echo "Error: -g argument must be gui (default), sdl, or" \
-                        "headless"
+                echo "Error: -g argument must be gui, sdl, or headless"
                 exit
             fi
             ;;