Allow script_args to be passed to deploy.sh

Resolves #218. Sample configuration might look like:

aws-amazon:
    provider: aws
    image: ami-1624987f
    size: Micro Instance
    ssh_username: ec2-user
    script: bootstrap-salt-minion
    script_args: -c /tmp/

This will help save some users from having to write wrapper scripts, just to pass options along to salt-bootstrap.

Tagging @s0undt3ch, @akoumjian.