Rename key_no_rotate to rotate_aes_key

A few additions and fixes for the key_no_rotate option.

Renames the option to 'rotate_aes_key'. Boolean-options should not have 
'true/false' or 'yes/no' in their name. It gets really confusing.

Currently 

```yaml
'key_no_rotate': True
```

actually rotates they key when deleting a minion-key.

Renaming it also adds clarity on what key exactly is being rotated.

Also adds documentation to master configuration reference that was missing.