Added base64 decoding of user data for OpenNebula.
change 'default' encoding to be "None"

Instead of just trying to see if userdata decodes as the indication that
it should be encoded, the user must explicitly set this.

The "just try it" will fail in the case where the user had other use
of user-data and wanted a blob of data to go through unrecognized by
cloud-init.

In cases where there can be mistake in automatic behavior,
and some users may be relaying on old behavior, its best to just require
explicit use.
OpenNebula: support base64 encoded user-data

This change adds the possibility to have base64 encoded userdata in
OpenNebula source.

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.

The change here allows the user to provide a second variable
USERDATA_ENCODING (or USER_DATA_ENCODING) and set that value to 'base64'
to indicate that USERDATA is base64 encoded.