Allow user_data in configuration

In some cases, it's useful to pass user_data to an AMI to tweak it prior to vagrant's configuration.  In my particular case, the problem was that /etc/sudoers in the AMI included "Default requiretty", which caused vagrant sudo commands to fail.  This patch, together with an extra config line:

aws.user_data = "#!/bin/sh\nsed -i -e 's/^\\(Defaults.*requiretty\\)/#\\1/' /etc/sudoers\n"

worked around the problem nicely.