Allow custom tag value delimiter

[erez-rabih]

Currently, tag values are delimited only by comma.
For example, if I have a server with the Roles tag set to web,db this server will have both roles.
If I use a different delimiter, ; for example, then the web;db will be treated as a single role.

I want to be able to set my delimiting factor as a configuration.
My use case is that I use knife-ec2 to spawn servers. When you pass tags to knife-ec2 they are comma delimited which makes it impossible to create tag values with commas in them, so I use ; as delimiter.