Deprecate local grub2 element

Upstream diskimage-builder has merged a version of the grub2 element,
so we don't need this one anymore and it conflicts with the upstream
version.  Rename it to grub2-deprecated until we can switch to the
upstream element.

Change-Id: I264d2ecfceed730ccf396e1c7338d4c670a4466a

elements/grub2-deprecated/README.rst

@@ -1,4 +1,7 @@
-Install grub2
+DEPRECATED: Install grub2
+
+Upstream has merged their own grub2 element, so this one will be going away
+as soon as all of the dependencies have been updated appropriately.
 
 Normally baremetal images have the bootloader removed because they boot from
 PXE-provided kernels and ramdisks, but localboot support requires grub2 to be

scripts/instack-build-images

@@ -135,7 +135,7 @@ puppet-modules \
 hiera \
 os-net-config \
 stable-interface-names \
-grub2 \
+grub2-deprecated \
 -p python-psutil,python-debtcollector \
 "