vcd_version should include terraform-provider-vcd version

[dataclouder]

The data source vcd_version can show the VCD version and the API version.
It can also trigger a check on a particular VCD version, which makes it useful to avoid running a script on a VCD that is not suitable for the features we want to run.

Similarly, the same data source could include a provider_version (returning the contents of the VERSION file), and a similar check, so that the data source would trigger an error if the requested provider version is not present.