Make vagrant scripts work with bash 3

Fixes #1890
Nitpick: could drop the leading space. Could also drop the `${...}` in NUM_MINIONS since `((...))` expressions accept variables without the $ syntax.

```shell
for ((i=0; i < NUM_MINIONS; i++)) do
```

But it's fine as it is, so no need to change it if you don't want to retest it.