elements: zypper-minimal: Install tar package

tar is an essential package but nothing pulls it explicitly. This causes
some issues in the openSUSE CI jobs like the following one

"Failed to execute tar: No such file or directory", "Failed to write
file: Broken pipe", "Failed to retrieve image file. (Wrong URL?)",
"Exiting."], "stdout": "", "stdout_lines": []}

Just like 'sed', add 'tar' to the list of packages for the openSUSE
minimal builds.

Change-Id: Ia36e3d9fd6b78862a6831ba80b43d4614a349ca0

diskimage_builder/elements/zypper-minimal/package-installs.yaml

@@ -11,3 +11,4 @@ lsb-release:
 openssl:
 sed:
 sudo:
+tar: