clean mbr section on all disks when installing os

Change-Id: I4eef54018b81ae62e4dff5cad89933537044d58f
Close-Bug: 1398630

cobbler/snippets/kickstart_pre_partition_disks

@@ -43,6 +43,9 @@ let found_disk_offset=0
 while [ \$disk_offset -lt \$disk_nums ]; do
     let found_disk=0
     disk_name=\$(basename \$1)
+#if $getVar('keep_old_partitions', '0') == "0"
+    dd if=/dev/zero of=/dev/\${disk_name} bs=512 count=1
+#end if
     if [[ x"\${disk_mapping[\${disk_name}]}" == x"" ]]; then
         echo "ignore disk \$1 since it is not in disk_mapping" >> /tmp/log
     	let disk_offset=\$disk_offset+1
@@ -103,6 +106,9 @@ done
 echo "clearpart --all --initlabel" > /tmp/part-include
 
 #if $getVar('keep_old_partitions', '0') != "0"
+for disk in \${sorted_disks[@]}; do
+    dd if=/dev/zero of=\${disk} bs=512 count=1
+done
 echo "only partition \$sorted_disks_str" >> /tmp/log
 echo "ignoredisk --only-use=\$sorted_disks_str" >> /tmp/part-include
 #end if

cobbler/snippets/preseed_pre_partition_disks

@@ -51,6 +51,9 @@ for disk in \$(list-devices disk); do
         continue
     fi
     disk_name=\$(basename \$disk)
+#if $getVar('keep_old_partitions', '0') == "0"
+    dd if=/dev/zero of=\$disk bs=512 count=1
+#end if
     eval "disk_path=\\${disk_\${disk_name}}" 
     if [ -z \${disk_path} ]; then
         echo "ignore disk \${disk_name} since it is not in disk_mapping" >> /tmp/preseed.log
@@ -129,6 +132,9 @@ done
 echo "first disk \${first_disk_name}" >> /tmp/preseed.log
 
 #if $getVar('keep_old_partitions', '0') != "0"
+for disk_name in \${sorted_disks}; do
+    dd if=/dev/zero of=/dev/\${disk_name} bs=512 count=1
+done
 remove_disks=\${sorted_disks}
 #else
 remove_disks=\${disks}