Allow to use a script for formating operations

• The script is not mandatory, if none is specify, kadeploy does what it was doing before
• One script for the 3 formating operation (deploy, tmp, swap) controlled via env variable
• The proposed script reuse UUID for swap and tmp partition since they are used on fstab and shared between prod & deploy env on grid'5000

lib/kadeploy3/server/config.rb

@@ -848,6 +848,7 @@ module Configuration
     attr_reader :decompress_environment
     attr_reader :group_of_nodes #Hashtable (key is a command name)
     attr_reader :partitioning_script
+    attr_reader :formating_script
     attr_reader :kexec_server_precmd
     attr_reader :kexec_script
     attr_reader :bootloader_script
@@ -982,6 +983,8 @@ module Configuration
           end
           @partitioning_script = cp.value('script',String,nil,
             { :type => 'file', :readable => true, :prefix => Config.dir() })
+          @formating_script = cp.value('formating_script',String,'',
+            { :type => 'file', :readable => true, :prefix => Config.dir() })
         end
 
         cp.parse('kexec', true) do