Remove BaseDRBD

BaseDRBD was probably useful when DRBD 0.7 and DRBD 8 were supported.
However, there is only one subclass of BaseDRBD remaining (DRBD8), and
the separation of responsibilities between those two classes was rather
randomly chosen.

The unification into one class also eases the introduction of
DRBD 8.4 support, as the responsibilities of the class(es) will be
distributed anew.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>