Add 'seed_master' feature and flag

Adding 'seed_master' feature to supersede 'accept_keys'.

Flag is defined as so:

```ruby
salt.install_master = true
salt.seed_master = {minion: "salt/key/minion.pub"}
```  

After bootstrap, the provisioner will check for un-accepted keys and then accept by name or 
upload any key defined in `salt.seed_master` to /etc/salt/pki/master/minions/.   

I made some minor changes to `salt.accept_keys` also:
- if keys are found in the master, it skips accepting keys.  This allows for repeated runs of `vagrant provision`
- a deprecation warning informs user of `salt.seed_master`