Allow passing options to tsuru now

So you can do stuff like:

  $ TSURU_NOW_OPTIONS="--tsuru-from-source" vagrant up

  $ vagrant ssh
  vagrant@vagrant-ubuntu-trusty-64:~$ go version
  go version go1.4 linux/amd64

  vagrant@vagrant-ubuntu-trusty-64:~$ ls -l ~/go/src/github.com/tsuru
  total 32
  drwxr-xr-x  3 vagrant vagrant 4096 Dec 23 17:11 commandmocker
  drwxr-xr-x  4 vagrant vagrant 4096 Dec 23 17:12 config
  drwxr-xr-x  6 vagrant vagrant 4096 Dec 23 17:12 docker-cluster
  drwxr-xr-x 16 vagrant vagrant 4096 Dec 23 17:13 gandalf
  drwxr-xr-x  3 vagrant vagrant 4096 Dec 23 17:13 go-gandalfclient
  drwxr-xr-x 30 vagrant vagrant 4096 Dec 23 17:08 tsuru
  drwxr-xr-x  6 vagrant vagrant 4096 Dec 23 17:16 tsuru-admin
  drwxr-xr-x  6 vagrant vagrant 4096 Dec 23 17:16 tsuru-client

Closes #9 

Cc: @fsouza