Automatically Generate Private Key

Allow Wireguard to automatically generate a private key if an
interface doesn't have one and private_key is set to auto. This
allows for auto key generation and distribution if using salt mines.

_states/wireguard.py

                 new=fwmark,
         )
 
+    if private_key == 'auto':
+        private_key = show.get('private key')
+        if private_key is None:
+            private_key = __salt__['wg.genkey']()
+            ret['changes']['private key'] = 'private key generated.'
+
     if show.get('private key') != private_key:
         __salt__['wg.set'](name, private_key=private_key)
         ret['changes']['private key'] = 'private key changed.'