Extract command generation logic from DRBD8

In order to easily support the DRBD 8.4 command syntax, this patch
extracts all command generation logic (i.e. where calls to `drbdsetup`
are assembled) into a new class DRBD83CmdGenerator.

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