Add support for default content deploy and add clear cache drush command

[Thommusa]

Here the patch to add 3 commands :
-drush cache-clear drush => need to clear cache command when rebuild
-set content dire : needs for default content deploy module
-import content : needs for default content deploy module

I send the complete build.xml but also default content deploy is a contrib module so maybe it's not generic to put content-import & set-content-dir in default build.xml
drush-dcd.patch.txt