Fix role for add_node method
Fix role for add_node method

Change default role for add_node method to 'fuel_slave'.
'add_node' is used by shell without passing 'role' attribute,
as result slave is created with wrong role.

Change-Id: I33de879c3a726866c8341fc129a678048324942d

stackforge/fuel-devops,stackforge/fuel-devops