ec2 module accepts user_data_file param

While the boto library doesn't support it, most other ec2 libraries and command line tools provide an option to specify a file which contains user data, instead of providing it as a string. This patch adds this capability to the ec2 module.

I've set the user_data_file's version_added to "1.7". I wasn't sure what it should be set to, so can change this if I guessed wrongly.