Add command generator for DRBD 8.4

The DRBD84CmdGenerator class, which generates commands suited for DRBD
8.4, is added. A common baseclass for DRBD83CmdGenerator and
DRBD84CmdGenerator is introduced as well.

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