Fix contrib/vagrant/Makefile to not ignore ENV['DEIS_NUM_INSTANCES'].

The current `contrib/vagrant/Makefile` ignores the value of `ENV['DEIS_NUM_INSTANCES']` and systematically uses 3 instances. This PR tells the Makefile to use the current value of `ENV['DEIS_NUM_INSTANCES']` or default to 3 instances if the environment variable is not set.

Before:
```sh
$ export DEIS_NUM_INSTANCES=2
$ make build # <--- incorrect, we expect to iterate over 2 instances only, not 3
i=1 ; while [ $i -le 3 ] ; do echo vagrant ssh deis-$i -c 'cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done' ; i=`expr $i + 1` ; done
vagrant ssh deis-1 -c cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done
vagrant ssh deis-2 -c cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done
vagrant ssh deis-3 -c cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done
```

After:
```sh
$ export DEIS_NUM_INSTANCES=2
$ make build # <-- expecting to iterate only twice
i=1 ; while [ $i -le 2 ] ; do echo vagrant ssh deis-$i -c 'cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done' ; i=`expr $i + 1` ; done
vagrant ssh deis-1 -c cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done
vagrant ssh deis-2 -c cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done
$ unset DEIS_NUM_INSTANCES   
$ make build # <-- expecting to iterate 3 times (default value)
i=1 ; while [ $i -le 3 ] ; do echo vagrant ssh deis-$i -c 'cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done' ; i=`expr $i + 1` ; done
vagrant ssh deis-1 -c cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done
vagrant ssh deis-2 -c cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done
vagrant ssh deis-3 -c cd share && for c in registry logger database cache controller builder router; do cd $c && docker build -t deis/$c . && cd ..; done
```