user-data on OpenNebula restricted to shell sourceable content

OpenNebula uses a text file with shell variables for storing the configuration variables (including user provided data). Some user data may not be renderable into this format, so using base64 encoding alleviates the problem. This change considers that, by default, USERDATA variable contains a base64 encoded string with the user provided data. This behavior can be deactivated by defining the USERDATA_ENCODING variable to anything different to "base64" (similarly to the Azure data source).

This feature is of special interest for resource providers with rOCCI-server front-end as the ones in EGI's Federated Cloud (https://wiki.egi.eu/wiki/Fedcloud-tf:Main)