Add RST version of ganeti-cleaner man page

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

man/ganeti-cleaner.rst

+ganeti-cleaner(8) Ganeti | Version @GANETI_VERSION@
+===================================================
+
+Name
+----
+
+ganeti-cleaner - Ganeti job queue cleaner
+
+Synopsis
+--------
+
+**ganeti-cleaner**
+
+DESCRIPTION
+-----------
+
+The **ganeti-cleaner** is a periodically run script to clean old job
+files from the job queue archive and to remove expired X509
+certificates and keys.
+
+**ganeti-cleaner** automatically removes all files older than 21 days
+from ``@LOCALSTATEDIR@/lib/ganeti/queue/archive`` and all expired
+certificates and keys from ``@LOCALSTATEDIR@/run/ganeti/crypto``.