Fix bug in bdev when drbd version format is x.x.x.x

This patch fixes a bug reported in [0]. Newer drbd versions can have
another digit beyond the regular major, minor and point release digits.
We modify the regex used to match that with an optional parts which is
not saved.

Furthermore it adds unittests that test for these different cases. Now
the data read from /proc is passed into the _GetVersion method, which
makes testing easier.

[0] http://code.google.com/p/ganeti/issues/detail?id=110



Signed-off-by: Manuel Franceschini <livewire@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>