createrepo: Add option to use xz compression

This patch adds a new config option createrepo_use_xz, which when set to
true will cause createrepo to compress sqlite databases with xz. The
default setting is False.

Fixes: #387
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

doc/configuration.rst

@@ -362,6 +362,10 @@ Options
     (*bool*) -- generate delta RPMs against an older compose. This needs to be
     used together with `--old-composes`` command line argument.
 
+**createrepo_use_xz** = False
+    (*bool*) -- whether to pass ``--xz`` to the createrepo command. This will
+    cause the SQLite databases to be compressed with xz.
+
 **product_id** = None
     (*scm_dict*) -- If specified, it should point to a directory with
     certificates ``<variant_uid>-<arch>-*.pem``. This certificate will be