Handle closing by window manager (:cancel event) (bnc#891550)

- the correct UI symbol is `:cancel`, not 
`:close`

- `:cancel` behaves like `:abort`
- setting `FAKE_BASE_PRODUCT` environment variable enables
  mocking SLES12 base system for easy testing on non-SLES12 systems
- 3.1.100
To simplify code a bit, either make this unconditional
```ruby
  when :cancel, :abort
    ...
    ret = :abort
```

or move it right after UserInput
```ruby
ret = Yast::UI.UserInput
ret = :abort if ret == :cancel
while ...
  case ret
  ...
  when :abort
```

Damn, I am nitpicking again.