Parse double protocol version in drbd8.2

DRBD 8.2 uses a double integer field ad protocol version, rather than a single
one. This patch fixes the ganeti parsing code, allowing both the old and the
new version type. In order to do so the internal _GetVersion function is
changed to return a dict, rather than a list, and the second protocol field is
added, only if present, as proto2.

This is a fix for issue 24.

Reviewed-by: iustinp