Add lvm storage option to gnt-cluster man page

This patch adds the previously introduced lvm storage toggling
info to the gnt-cluster man page. It adds some info the the
gnt-cluster init -g option and adds the gnt-cluster modify command.

Reviewed-by: iustinp

man/gnt-cluster.sgml

@@ -214,6 +214,7 @@
         <arg>-g <replaceable>vg-name</replaceable></arg>
         <arg>--master-netdev <replaceable>vg-name</replaceable></arg>
         <arg>-m <replaceable>mac-prefix</replaceable></arg>
+        <arg>--no-lvm-storage</arg>
         <arg choice="req"><replaceable>clustername</replaceable></arg>
       </cmdsynopsis>
 
@@ -270,8 +271,13 @@
 
       <para>
         The <option>-g</option> option will let you specify a volume group
-        different than xenvg for ganeti to use when creating instance disks.
-        This volume group must have the same name on all nodes.
+        different than 'xenvg' for ganeti to use when creating instance disks.
+        This volume group must have the same name on all nodes. Once the
+        cluster is initialized this can be altered by using the
+        <command>modify</command> command. If you don't want to use lvm
+        storage at all use the <option>--no-lvm-storage</option> option.
+        Once the cluster is initialized you can change this setup with the
+        <command>modify</command> command.
       </para>
 
       <para>
@@ -288,6 +294,13 @@
         default is aa:00:00.
       </para>
 
+      <para>
+        The <option>--no-lvm-storage</option> allows you to initialize the
+        cluster without lvm support. This means that only instances using
+        files as storage backend will be possible to create. Once the cluster
+        is initialized you can change this setup with the
+        <command>modify</command> command.
+      </para>
     </refsect2>
 
     <refsect2>
@@ -312,6 +325,25 @@
       </para>
     </refsect2>
 
+    <refsect2>
+      <title>MODIFY</title>
+
+      <cmdsynopsis>
+        <command>modify</command>
+        <arg choice="opt">-g <replaceable>vg-name</replaceable></arg>
+        <arg choice="opt">--no-lvm-storage</arg>
+      </cmdsynopsis>
+
+        <para>
+          Modify the options for the cluster.
+        </para>
+
+        <para>
+          The <option>-g</option> and <option>--no-lvm-stoarge</option> are
+          described in the <command>init</command> command.
+        </para>
+    </refsect2>
+
     <refsect2>
       <title>REMOVE-TAGS</title>