Correct argument variables implementation.

[nrbnlulu]

Currently all arguments are optional though you must set them (what?!?).
In order to do this correctly we should:

• get the default value of the argument (schema level)
• if operation overrides some argument value (without an operation variable) set it as the default for this operation
• if a operation variable is applied on an argument if the argument has a default make it the operation variable default otherwise
  the operation variable has no default and must be set explicitly.