Introduce new Pithos variables

Introduce new Pithos variables for selectable storage backend.

snf-image-host/defaults.in

@@ -50,13 +50,29 @@
 # MULTISTRAP_APTPREFDIR="@MULTISTRAP_APTPREFDIR@"
 
 # XEN_SCRIPTS_DIR: Directory where the Xen scripts are stored
-# XEN_SCRIPTS_DIR=="@sysconfdir@/xen/scripts"
+# XEN_SCRIPTS_DIR="@sysconfdir@/xen/scripts"
 
 # PITHOS_DB: Pithos database in SQLAlchemy format
 # PITHOS_DB="sqlite:////@localstatedir@/lib/pithos/backend.db"
 
+# PITHOS_BACKEND_STORAGE: Select Pithos backend storage. Possible values are
+# 'nfs' and 'rados'. According to the value you select, you need to set the
+# corresponding variables that follow.
+# If you select 'nfs' that's 'PITHOS_DATA'. If you select 'rados' then you need
+# to set all the "*_RADOS_*" ones.
+# PITHOS_BACKEND_STORAGE="nfs"
+
 # PITHOS_DATA: Directory where Pithos data are hosted
 # PITHOS_DATA="/@localstatedir@/lib/pithos/data"
+#
+# PITHOS_RADOS_CEPH_CONF: RADOS configuration file
+# PITHOS_RADOS_CEPH_CONF="@sysconfdir@/ceph/ceph.conf"
+
+# PITHOS_RADOS_POOL_MAPS: RADOS pool for storing Pithos maps
+# PITHOS_RADOS_POOL_MAPS="maps"
+
+# PITHOS_RADOS_POOL_BLOCKS: RADOS pool for storing Pithos blocks
+# PITHOS_RADOS_POOL_BLOCKS="blocks"
 
 # PROGRESS_MONITOR: External program that monitors the progress of the image
 # deployment. The snf-image monitor messages will be redirected to the standard