Update mkimage-yum.sh

If you want to makeimage using the group "Compute Node" and so on, you must add “ ” to include the $install_groups, or it will format the text as below:
yum -c /etc/yum.conf --installroot=/tmp/makeimage.sh.zOLs8y --releasever=/ --setopt=tsflags=nodocs --setopt=group_package_types=mandatory -y groupinstall Compute Node
That's absolutely incorrect.

Change-Id: I8b6b09f215aabd6b1f76c9365ba96c68722c47fd
Signed-off-by: dodia <tangwj2@lenovo.com>

contrib/mkimage-yum.sh

@@ -81,13 +81,13 @@ fi
 if [[ -n "$install_groups" ]];
 then
     yum -c "$yum_config" --installroot="$target" --releasever=/ --setopt=tsflags=nodocs \
-        --setopt=group_package_types=mandatory -y groupinstall $install_groups
+        --setopt=group_package_types=mandatory -y groupinstall "$install_groups"
 fi
 
 if [[ -n "$install_packages" ]];
 then
     yum -c "$yum_config" --installroot="$target" --releasever=/ --setopt=tsflags=nodocs \
-        --setopt=group_package_types=mandatory -y install $install_packages
+        --setopt=group_package_types=mandatory -y install "$install_packages"
 fi
 
 yum -c "$yum_config" --installroot="$target" -y clean all