Convert gnt-os man page to sections

As Constantinos observed, the gnt-os man page is not consistent with the
others. This patch adds the missing sections.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

man/gnt-os.rst

@@ -20,7 +20,10 @@ system flavours for the instances in the Ganeti cluster.
 COMMANDS
 --------
 
-**list**
+LIST
+~~~~
+
+**list** [\--no-headers]
 
 Gives the list of available/supported OS to use in the instances.
 When creating the instance you can give the OS-name as an option.
@@ -28,6 +31,9 @@ When creating the instance you can give the OS-name as an option.
 Note that hidden or blacklisted OSes are not displayed by this
 command, use **diagnose** for showing those.
 
+DIAGNOSE
+~~~~~~~~
+
 **diagnose**
 
 This command will help you see why an installed OS is not available
@@ -37,6 +43,9 @@ missing from a node, or is only partially installed, and this
 command will show the details of all the OSes and the reasons they
 are or are not valid.
 
+INFO
+~~~~
+
 **info**
 
 This command will list detailed information about each OS available
@@ -44,6 +53,9 @@ in the cluster, including its validity status, the supported API
 versions, the supported parameters (if any) and their
 documentations, etc.
 
+MODIFY
+~~~~~~
+
 | **modify** [\--submit] [-H *HYPERVISOR*:option=*value*[,...]]
 | [\--hidden=*yes|no*] [\--blacklisted=*yes|no*]
 | {*OS*}