Security group id patch

A single security group given as a string i.e. knife[:security_group_ids]= 'sg-0172f169'
fails when specified in knife.rb. A patch has been applied for this in ec2 server create.
I'd like to avoid compat issues here where one version is ok with strings, older versions aren't, as well as inconsistency with other command line options. I think a compromise is to simply have a better error message -- if they don't specify an array of strings, give an error message that says that this must be specified as an array. The user will quickly be on her way since we will have given her actionable feedback.