Don't validate an option in ejabberd_config:get_option() functions

Don't validate an option in ejabberd_config:get_option() functions

The commit introduces the following changes:
* Now there is no need to pass validating function in
  ejabberd_config:get_option() functions, because the configuration
  keeps already validated values.
* New function ejabberd_config:get_option/1 is introduced
* Function ejabberd_config:get_option/3 is deprecated. If the function
  is still called, the second argument (validating function) is simply
  ignored.
* The second argument for ejabberd_config:get_option/2 is now
  a default value, not a validating function.