The logic around the DRBD usermode helper so far was that
setting it was only possible when DRDB was not explicitely
disabled. This patches changes it in a way that it is
consistent to how Ganeti handles the volume group name.

Now, the user can specify a DRBD usermode helper independent
of whether or not DRBD is enabled or not. She will however
get a warning when she sets a helper without having DRBD
enabled. The reasoning behind this is that one might want
to configure a helper while not yet having set up DRBD
completely or while having DRBD disabled temporarily without
loosing this piece of configuration.

This change was done earlier in the patch series, because
I wanted to do the refactoring in two steps, first
just transforming the original logic from --no-drbd-storage
to --enabled-disk-templates and if that goes well, adjust
to the more user-friendly behavior.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>