more metadata options

We use this for our cloudformation scripts so why not share

* more options added for InitFile, made content optional
* added InitService and InitServices with an example of using it
This isn't an actual function in CloudFormation right?  I'm trying to figure out what the right way to handle this is - at the very least, if it's not (I couldn't find it) we probably want a docstring detailing that this isn't part of the CloudFormation spec, and is just a helper function that is part of troposphere alone.