[LIBCLOUD-726] Added support for custom data in create_node function …

[davidwilson2038]

Azure has support for Custom Data (i.e. "User Data") in base64 format to be included on node creation (https://msdn.microsoft.com/en-us/library/azure/jj157194.aspx). I have implemented this in the create_node function for Linux VMs. This is an important feature for any workflows that rely on cloud-init.

[Kami]

Thanks.

I've pushed some change and merged your PR intro trunk.

1. Explicitly declare the method argument, add missing docstring - d79cffd
2. Don't require user to base64 encode the value and do the encoding inside the method - cf32c27