Forward-port DrbdNetReconfig

This is a modified forward-port of DrbdNetReconfig and their associated
RPCs. In Ganeti 2.0, these functions will be used for two things:
  - live migration (as in 1.2)
  - and for other network reconfiguration tasks, since DRBD8.Attach()
    doesn't do them anymore

Because of the Attach() changes, we can now implement the
AttachNet/DisconnectNet functions as independent entities, and we don't
need the cache anymore.

Note these functions are copies of the latest 1.2 code, and not
cherry-picks of the (many) patches that went into 1.2.

Reviewed-by: ultrotter