[2.3][Form] renamed option "always_empty" to "reset_on_submit"

Bug fix: no
Feature addition: yes
Backwards compatibility break: yes
Symfony2 tests pass: yes
Fixes the following tickets: #6325
Todo:
- [ ] update documentation 
License of the code: MIT
Documentation PR: -

This PR renames the option "always_empty" to "reset_on_submit" for more clarity (the old option is deprecated and usable until 2.3).