Add zypper element

This provides some customizations for zypper based distributions (e.g.
openSUSE and SLES). It is heavily inspired by the yum element and sets up
zypper to cache downloaded package outside of the chroot so that they can be
reused to speed up subsequent builds.

Change-Id: I775c921ee19cf7d1180fb68c1b7857ea6245a77d

elements/opensuse/element-deps

@@ -1,2 +1,3 @@
 cache-url
 dib-run-parts
+zypper

elements/zypper/README.md

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+This element provides some customizations for zypper based distributions like
+SLES and openSUSE. It works in a very similar way as the yum element does for
+yum based distributions.
+
+Zypper is reconfigured so that it keeps downloaded packages cached outside of
+the build chroot so that they can be reused by subsequent image builds. The
+cache increases image building speed when building multiple images, especially
+on slow connections.  This is more effective than using an HTTP proxy for
+caching packages since the download servers will often redirect clients to
+different mirrors.

elements/zypper/post-install.d/99-zypper-no-keep-packages

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+#!/bin/bash
+
+set -e
+
+# Disable caching packages for all repositories
+sudo zypper modifyrepo --all --no-keep-packages

elements/zypper/pre-install.d/01-zypper-keep-packages

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+#!/bin/bash
+
+set -e
+
+# Enable caching packages for all repositories
+sudo zypper modifyrepo --all --keep-packages

elements/zypper/root.d/50-zypper-cache

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+#!/bin/bash
+
+set -e
+
+[ -n "$TARGET_ROOT" ]
+
+ZYPPER_CACHE_DIR=~/.cache/image-create/zypper
+mkdir -p $ZYPPER_CACHE_DIR
+
+sudo mount --bind $ZYPPER_CACHE_DIR $TMP_MOUNT_PATH/var/cache/zypp