fix recursive default variables

Only the travis and development version use non-default values for securedrop_code and securedrop_user.

Instead of defining a default value ever location that securedrop_code or securedrop_user added two variables non_default_securedrop_code and non_default_securedrop_user. If these variables are defined (in travis-specific.yml and development-specific.yml) then those values will be used for securedrop_code and securedrop_user respectively.