Add an ignoredisk directive to the partitionfile

When installing, explicitly tell anaconda to only manipulate
$instdisk.  Anaconda tends to be too aggressive about disks.

xCAT-server/share/xcat/install/scripts/pre.rh.rhels7

@@ -175,6 +175,7 @@ if uname -r|grep -q '^3.*el7'; then
     EFIFSTYPE=efi
 fi
 
+echo "ignoredisk --only-use=$instdisk" >> /tmp/partitionfile
 if [ `uname -m` = "ppc64" -o `uname -m` = "ppc64le" ]; then
 	echo 'part None --fstype "PPC PReP Boot" --ondisk '$instdisk' --size 8' >> /tmp/partitionfile 
 fi