Make vagrant scripts work with bash 3

Fixes #1890
I think this would be simpler:

```shell
  local ip_last_octet=${ip##*.}
```

Or if you really want to do validation that it starts with MINION_IP_BASE (I don't think that's really necessary):

```shell
  local ip_last_octet=${ip#${MINION_IP_BASE}}
```

And then:

```shell
  local minion_number
  minion_number=$((ip_last_octet - 1))
  echo "minion-${minion_number}"
```

WDYT?