dom0-update: prevent template package upgrades (#996)

This would override user changes to the template. Previous method
(Obsoletes: rpm header) doesn't work now, so add explicit exclude list
as yum option

Fixes qubesos/qubes-issues#996

dom0-updates/qubes-dom0-update

@@ -22,11 +22,13 @@ if [ "$1" = "--help" ]; then
     exit
 fi
 
+# Prevent template upgrade - this would override user changes
+TEMPLATE_EXCLUDE_OPTS="--exclude=`rpm -qa --qf '%{NAME},' qubes-template-\*`"
 PKGS=
-YUM_OPTS=
+YUM_OPTS="$TEMPLATE_EXCLUDE_OPTS"
 GUI=
 CHECK_ONLY=
-ALL_OPTS=$*
+ALL_OPTS="$TEMPLATE_EXCLUDE_OPTS $*"
 YUM_ACTION=
 QVMRUN_OPTS=
 CLEAN=