add an option to set user data
s/:user_data/:cloudstack_user_data/g
assign the value

i'm so bad at programming
do base64 content of user-data

instead of fixing fog, do base64 in knife-cloudstack because it breaks
backward compatibility.
use Base64.strict_encode64

it turned out that 2.2.15 does not like new line in encoded strings.
Merge pull request #17 from reallyenglish/support-user-data

Support user data for cloudstack